I replied that if he can find a retaining ring. Then he asked where to find one, that brings me on the topic, there a lot of LTM to M adapter, or LTM to M43 (digital camera) or M42 to M43 adapters in market, thanks to interchangeable digital cameras.
If find one as retaining ring, then SL66 lens adapter could mount any LTM or M42 lenses!
Stanley send me three sets of M42 to M43 adapters, each attached with a 42-39mm adapter, so this adapter would take M42 lenses or LTM lenses.
I lathe the SL66 lens adapter hole into proper size, so this adapter could tightly sit on top. glued and drilled three screw hole by side, to make sure two adapters are now in one.
Bring this adapter back home, mount a 39mm thread lens, seems it fit alright.
But when I tried to un-mount the lens, it seems that adapter so fit on the camera body, it simply refused to depart, I had to un-screw the lens first, then I can un-mount the adapter. It is because the adapter is flat, no place for hands to rotate it to un-mount, tomorrow I will change those 3 tiny screws into inner-hexa-head, that will provide good support for un-mount.
Another LTM lens, but I know it is not proper for SL66, this lens only got 25mm focal length, I just want to see if a LTM lens works on adapter or not, it fits.
Maybe I should find a Hektor or Elmar 135mm lens, to see how it make medium format image.
Replace flathead screws into hex socket cap screws, thus makes un-mounting adapter much easier.