I put a thinner caseback on mine so yeah it was sourced somewhere at some time
Great idea, I found a case back off a scrap YM with a 2824 movement and that fit perfectly!! In fact I had to readjust clasp/strap one hole in cause is was so much thinner.
Before
After
You might remember earlier in this post I mentioned trying to fit a case back from my gen Explorer II 16570 and it was too small for the BP case. I'm wondering now if one of the reasons might be cause ETA movement cases are larger than gen style cases???