Just to add what I've learned so far:
I had heard for years that the difference between a Springy and a Goldstar trem block is that the Springy used a zinc alloy and the Goldstar used a steel block.
Further, I had heard that the Springy block was marked with circles and "Made In Japan" and the Goldstars were blank. But I have seen lots of marked blocks after 1983.
1977-'81 they seemed to have used a die-cast "inertia bar" (block) of unknown material, a weird amalgamation of the shape of the vintage Strat block and the "mazak" cast metal blocks CBS introduced in 1971. Complete with the 3 circles used on the Fender mazak blocks but of course, unlike Fender, still equipped with the bent steel saddles.
I believe the transition point to milled steel inertia bars is 1982 and the introduction of the "Final Pro.spec" hardware. So there are a few Springys with that bridge.
Also the block was one unit with the bridge in the beginning then was later changed to a detachable design. Not sure when exactly that happened.
I never noticed that before, but all pre-1982 "Final Pro.spec" Springy bridges seem to lack the 3 flathead Phillips screws tightening the inertia bar to the chrome base plate on the originals. Only the "Final Pro.spec" bridges have them (?):
So it seems they are all glued or welded together somehow and hence "one part" until 1982. Here's a base plate from an "S.T.C." bridge:
Speaking of which, these low-cost bridges seem very similar to the regular 77-81 Springy bridges in all aspects, apart from the die-cast saddles that can only be found on Silver Stars and ST-42s (?). These "S.T.C. P-II" bridges can be found on loads of cheaper MIJ Strat copies.
However, there are small differences between the pre-1982 Tokai cast metal blocks and the "S.T.C." hardware if you look closely - the "MADE IN JAPAN" is more embossed and at a slightly different position, with more space between the 'N' and the circle next to it, they also look more carefully made.