This one was a 50mm f1.8 too. Must be a slightly different design.
Yes, the slot is just a recess on the outer side of the helical so no gaps for light to get in. I thought it might be possible for some light to get down the side of the slot when it was wound right out but it seems to be masked fully. My mounting plate will further mask it leaving just a 42.5mm dia hole for the lens to poke through (it's actually flush with this plate at infinity).
I agree. So far, so good. Except I have just noticed that the three mounting holes on the front face are not equi-distant around the diameter like I drilled them. Oh well, I'll have another go!
I have just done some measuring.
Distance from the film plane to the front of the body: 45mm.
Distance from the back of the lens (front of lens board) to front of body (at infinity): 25.5mm.
Total distance: 70.5mm.
Schneider data sheet figure for film plane to back of lens: 70.5mm !!!
I was originally going to mount the helical mount on a plate and mount the plate on the body. Now I intend to mount it directly to the body but recess it by 3mm. This will hide the hyperfocal marks on the barrel (which will be wrong) and move the lens too close to the film. To counter this I will have a shim area to bring the film plane back to the correct place which I will find by using ground glass rather than an absolute measurement.
As the film gate and spool holders come out as one unit which locates on pins, it just needs a suitably sized spacer to butt up against to ensure the film distance is always correct.
Next week I will bite the bullet and start cutting the oak to make the body. I only have just enough for the body so I expect I will make a grey PVC body first or possibly one from 3/8" MDF.
Check out the other complex(?) screw mechanism in the outer part by getting under the rubber focusing ring. Its possible to move the helical lens part in and out without it turning the helical. I found this dead useful with fine adjusting for infinity while keeping the infinity point on the lens in the same place.