I can't quite remember what the exact layout/function is for the external drillings in the earlier gear case but about 1950 (I think) it changed to having only one, with the stud for the rocker feed, then 1952 on there were no external take-offs (although the bosses are still clearly there) and the rocker oil feed was taken from the return line.
The change Nev refers to is essentially cosmetic, at the top lug by the magneto the casting changes to be more angular. All interchangeable though and if you're good with the drilling and machining you could probably convert a later gear case to have the earlier oil take-offs.
Your case looks slightly odd but I think the rocker feed is the stud still intact at the bottom of your piccie and that hole with the crack is for the 'pressure regulator' which is only there to make the oil pressure gauge work and (as Nev notes) you can do without that, so if you can blank off and seal up that cracked part your gear case will still be OK.
You might find some info in this thread (also a picture of a gear case with only the rocker feed stud and just the boss where your cracked hole is)viewtopic.php?f=4&t=1708
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)