Baker stated he did not draw his gun
out until he reached the 1st staircase
That’s likely the front staircase
Pauline Sanders noticed Baker as he ran right up the right side of those entrance steps, because Pauline was standing just to right side of the entrance door.
It’s implausible imo that Pauline Sanders would have failed to mention a detail such as a gun drawn by the the cop she observed.
Therefore, I’m going with a meeting of Baker and Oswald happening in the front foyer = “vestibule “ at the front staircase as Oswald was seen going TO the storage room by Baker.
Oswald still had on his brown shirt at this point, although it’s uncertain if the sleeves were still rolled up or rolled down. Baker thought it was a jacket.
Truly then entered, saw Baker with gun drawn at Oswald and then vouched for Oswald and advised Baker to follow Truly to the rear elevators and staircase if Baker wanted to get to the roof.
Oswald went back to the storage room. took OFF his brown shirt and then Oswald followed the path of Baker/Truly going thru the now opened countertop but some 15 to 20 seconds later , such that Baker/Truly have ascended to the 2nd floor landing and out of LOS of Oswald.
Purpose of Oswalds trek across 1st floor to the rear staircase was to get another Dr. Peoper from the 2st floor machine.
However, the Dr,Peoper machine was either sold out or the power has gone out, and so Oswald goes up the rear staircase to go get a coke from the 2nd floor lunchroom.
He gets the coke, but dies not open it, and opts to return to the front foyer and storage room by cutting thru Mrs Reid’s office.
Thus the Mrs Reid meeting Oswald in T-shirt story thingy can be ipreserved “ “INTACT and SECURE” ! ( a phrase from “Fallout” episode of The Prisoner)