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!