I saw one of our Vice Presidents in the 1970s; there was security everywhere, on store tops and so forth near the speech at the County Courthouse. So, this is some time after 1963 but somewhere along the line, they may have used some walkie-talkies.
And thinking about that, brought me to thinking too....transistor radios were in the vogue around 1963. Now, looking at the pictures, I think it's more likely a walkie-talkie. I wonder if there was a live broadcast on the JFK motorcade? I don't think so. I'm just throwing out another possibility.
A walkie-talkie is rather long, even in 1963. Bulky? I don't know. The ones in WWII were bulky.