The common theory is that at the time the Silver Streakwas launched, the DB range was really only a 3 model range of full cromo f&f, supplied either built up or as kits/f&f sets.
So there was basically, 1 frame in 3 sizes. All the bikes were just badged "Diamond Back".
The Silver Streak is the first frame added to the range, as an attempt to bring prices down, to be sold alongside its full cromo brothers. It looks like a medium Diamond Back, but it isn't, as it's got a hi ten rear looptail witha cromo front triangle and thicker rear drop outs. So being the second basic frame design (rather than size) its logically the second in the range.
Or something.
Sparkys the DB expert over at os-db.net so ask him....
And btw, I had a SS bidt, chose it over a Torker 280, and didn't realise it wasn't full cromo until I started looking for a bmx to satisfy my childish cravings a few months ago. Both bikes very similar, but I presume the Torker would have been full cromo, and possibly then a slightly better bike for the money.
But with its black chrome colur scheme, neat welding, distinctive gusset, arayas, proper D/C brakes sugino stem and 1pc ect, the SS was a lovely bike to look at...