I'm just pulling apart a Mk I Stage 73 from week 35, 1973. Here is what I found:
Keys 1 - 25 All yellow springs, and this exactly corresponds with the tonebars that have the slot between the voicing screw and the quarter twist
Keys 25-53 All natural (unpainted) springs and this corresponds to tonebars with no slot and one quarter twist
Keys 54-73 Blue springs on the escapement (end) screw and natural springs on the voicing side.
This is contrary to the replacement parts above, but it makes sense that the springing would match the tonebar style change (for some reason??)
The spring wire diameter measures:
Yellow - 1.02 mm
Natural - 0.89 mm
Blue - 0.65 mm
This piano has mostly original parts...I don't think that it has ever been completely stripped down. I'm trying the McMaster tonebar screw and grommet replacements for the first time and it's working well.
As long as we're re-visiting this old thread, my June 1974 Mark I Stage (with tonebars marked 1-73) is as-described above, except that the all natural springs section extends through key 60. Starting at key 61, I have thinner blue springs for escapement (closest to the player) and thicker natural springs for voicing.
My piano appeared to have been in original (unteched) condition when I bought it a couple years ago. So, for the benefit of the archives, it seems like there was some variation by year. (Or by assembler.)
Alan