The stamped number on my '66 GS springs (different specs for '66) was 654B380.

That translates to 0.654" wire diameter and rate of 380 lbs/inch.

If your stamp is actually 675B435, that would be 0.675" wire diameter with rate of 435 lbs/inch

The specs I found for the 65 GS were 0.669" dia with rate of 420 lbs/inch...... that's real close to what you have, they could be original.

I don't know of anyone else here that found the stamping on their original springs.