I doubt very much that there are any internal differences between these boxes. I do believe, however, that the operating condition of these things after 30 years can vary greatly from one example to another so one comparison betwen a single robin vs bolt example shouldn't be taken as "truth."
I do know that I've looked at the amp in the Bolt boxes and have concluded that it uses an outrageously powerful amp (something like 20wpc, BTL). When working to it's potential, I find it hard to believe any boombox having noticeably more power. My guess..... the bolt used in the comparison might not have been at 100%.