Bumped for "Richard Smith":
There is no doubt that an "official" roll call did not take place.
At no time did Truly gather all his men together to discover that only Oswald was missing. We can tell this from Truly's comment:
"I didn't know whether they were all there or not."
BUT...
There was a time on the first floor when Truly's "boys"were being processed with the express intent of finding who was on the sixth floor that day. It was noticed by some of the men that Oswald was missing at that time:
EDDIE PIPER
MR. BALL -- Did you at any time after the shooting miss Lee Oswald—did you notice he wasn't around?
MR.PIPER -- No, sir; I didn't notice it until the lineup. You know, I just figured all the people was there.
MR. BALL -- You did notice it at the lineup, did you?
MR. PIPER -- Yes.
MR. BALL -- Tell us about that.
MR. PIPER -- I did notice it in the lineup.
MR. BALL -- What do you mean by the lineup?
MR. PIPER -- I mean, when they lined us all up and told us to give our name and address and just to go home.
MR. BALL -- You say "they"; who do you mean?
MR. PIPER -- The detective—whoever it was.
MR. BALL -- The police?
MR. PIPER -- Yes; they had the building all surrounded. They went to locking the doors back and front and told us to all come up and then go home, and I told him, I says, "I've got to go down in the basement and get my clothes," and he said, "You can go down and get your clothes and come on back up here, but give me your identification and your name and tell us where you are staying," and everybody heard me say that, I guess, and he let us out of the building, one by one, and I went on out the front door.
JAMES JARMAN
MR. JARMAN -- When we started to line up to show our identification, quite a few of us asked where was Lee. That is what we called him, and he wasn't anywhere around. We started asking each other, have you seen Lee Oswald, and they said no.
BONNIE RAY WILLIAMS
MR. WILLIAMS. -- When we arrived to the first floor, the first thing I noticed was that the policemen had rushed in. I think some firemen came in with a water hose. And then the next thing that happened, these detectives, or maybe FBI--anyway, they stopped us all and they said, "Do you work here?" And we told them yes. And they took our name, address, and they searched everybody. And then the other fellow--I think one fellow asked whether we had been working upstairs. I think we told him yes. They got out all the fellows I think that was working on the sixth floor at the time, and they took us all down to the courthouse, I think, and we had to fill out some affidavits and things.
It would appear that if there was anything resembling a roll call it would have been while they were being processed and it was noticed Oswald was missing, particularly, as Williams points out, the emphasis was on finding who had been working on the sixth floor. It is this "processing" Truly observes when he states:
"There were other officers in other parts of the building taking other employees, like office people's names. I noticed that Lee Oswald was not among these boys."
It would seem reasonable that this is the moment Truly noticed Oswald was missing and decided to take action.
However, as is often the case, things are never that simple.
Detective B. L. Senkel arrives at the TSBD about 12:50pm (With Det. F. M. Turner, Dep. Chief Lumpkin and Forrest Sorrels). He enters the rear door and starts searching floor by floor until he reaches the sixth floor where he finds Fritz, Sims and Boyd. He arrives there around the time the hulls are found in the southeast corner and before the rifle is found:
"Capt. Fritz advised me to take the employees that had been on the sixth floor to the City Hall for statements. Officer C. W. Brown stated he had a car and would drive me to City Hall. Brown and I left the Texas School Book Depository with witnesses W. H. Shelly, Bonnie Ray Williams, and Danny Garcia Arce."
Detective C. W. Brown also enters the TSBD by the rear door and makes his way up to the 6th floor where he contacts Fritz:
"Capt. Fritz advised me and Det. B. L. Senkel, who was already there, to bring the employees of this building to the Homicide Office and get affidavits from them."
Before the rifle is even found Detectives Senkel and Brown are down on the first floor processing the employees specifically trying to find who was on the sixth floor. As a result, around 1:30pm, Shelley, Williams and Arce are taken to City Hall. There is no mention of going back to collect any of the other sixth floor employees - Oswald, Givens, Lovelady and Dougherty. We know with hindsight Oswald has gone and Givens never returned, but what about Lovelady and Dougherty?
When Truly notices Oswald is missing he is with Shelley, so this must be the time when Dets. Senkel and Brown are processing the "boys" and before Shelley, Williams and Arce are taken to City Hall, approximately 1:20 - 1:30pm. It is well known with hindsight that Oswald and Givens are missing from this processing but what is less well known is that Lovelady and Dougherty are also missing.
In his HSCA interview Lovelady states that he takes a group of officers up through the TSBD to the seventh floor. He then takes them down to the 6th floor and is present when the rifle is found around 1:25pm.
Dougherty is also on the sixth floor but even later:
"Well, when the FBI men---I imagine it was who it was---he showed me his credentials, but he asked me who the manager was, and I told him, "Mr. Truly." He told me to go find him. Well, I didn't know where he was so I started from the first floor and Just started looking for him, and .by the time I got to the sixth floor, they had found a gun and shells."
At the time of "processing" Dougherty is wandering around the TSBD actually looking for Truly!
This is why Lovelady and Dougherty are not taken down to City Hall until later - because they are not present when the processing takes place. This is why Truly states that there were still employees missing when he decides that only Oswald is missing.
So how can he be so confident that Oswald has left the building?
I believe the answer comes from Oswald.
During his interrogations he reveals that before he leaves the TSBD he meets Shelley near the front entrance and it is on Shelley's advice that he actually leaves. If there is anything to this then Shelley is in a perfect position to tell Truly that Oswald has already left. Obviously, neither man says this is the case but it is the only way I can think of that Truly can be so certain that Oswald is missing.