That's the Bendix model number, not a date code. R2BT4 was documented in the June 1963 issue of SAMS Auto Radio series. The next Triumph-branded model, 4TBTR was documented in the September 1964 issue, so your radio was probably initially installed some time during those two years.
Note that it is actually a hybrid, with two tubes and two transistors. I had one like that in my 62 Chevy that said "SOLID STATE" across the face, but it still had to warm up
PS, you may know this already; there is a plug that you can install different ways to accommodate positive or negative ground. I don't see any reverse polarity protection, so connecting it backwards would probably be a Bad Thing.