Your measurements of 8.75 x 21mm put this at a ligne size of 3.75 x 9.25 (or perhaps 4 x 9.5). There aren't that many movements of this size and the most common is the FHF 59 (which other manufacturers used as well and gave their own movement ID to). Unfortunately it's not this as the keyless works and bridge layout are completely different to your movement.
Which brings me on to your probability of an ETA 651. That's a no ... at 10 x 22.7mm it's larger than yours and of course the keyless works (and bridge layout) are different. In my searching I had also come across the ETA 746 but it's the same story.
I have spent ages looking through stuff trying to find your movement but no luck. I've found setting lever springs that look similar but then a no-go on the movement size; a (Van) Buren movement looked promising but didn't tie up etc. etc.
I hope someone strikes lucky!
Hi Nickelsilver, Thank you for all the help. Will search for the tech sheet.
I have many used genuine hairsprings and balances that I have been vibrating together. Need to place order for more hairsprings of compatible springness.
Been trying to experimentally find a compatible HS of known CGS.
Strength is proportional to area of the coil outer circle.
I think I may find CGS direct out of this tables.
Will keep you posted. Thank you, great help. Best
Found one of the tables, it's in French but easy to figure out.
If doing the test spring process, the formula is K=K' (D/D') 2
K is the CGS number you're looking for
K' is the CGS of the spring you test
D is the diameter of spring you need, at the pinning point
D' is the diameter of the correctly vibrated test spring at the pinning point.
Sorry to the OP Joe and I are hijacking the thread a bit, maybe a mod could move it to a new thread?