That is a great guess and very well may be the case. But only 2 of the 4 that I found in the registry with "BT" are HCS cars. The other two are 71 and 72 DSOs. And I also find it really hard to believe that only 4 of the cars in the registry were submitted by Bob. He was constantly finding new cars hidden in the Colorado area.
There are a couple other mysterious letters after other cars, such as "T" (153983, 169938) and "M" (152456, 152705) No clue what those mean.
My guess is those letters were part of some shorthand that Paul used, but they really weren't intended to make it into the printed registry book. Paul is a pretty detail oriented guy. I think if he meant those to be there he would have included a footnote explaining them.