Vintage Minor Register

Not quite in the same league as Lee's mystery bolts, but can anyone tell me what the stud on the o/s/r of my block is for. Incidentally I presume that the numbers 4-2-31 on the block will be the casting date.

Regards,

Philip

This is the early type block that had a brass tap in this position for draining the coolant from here as well as the radiator tap.