Go on - be a devil.
I know a guy who ran two cars, one quite decent and the other an old banger. When he did an oil change on the good one he put the old oil in the banger.
I think I just put Hypoid 90 in mine - it's been in there for about 25 years! Do you think I should change it?
Oh, mine doesn't have that boss where your screw is in the casting....This is what I mean
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I wondered whether it should be lower down, but I can't see anything.
I've just taken the side panel off, and that screw I pointed out above is the drain plug..
The backlash seems, part of the design, nothing appears to be loose.
What does that cranked lever in the bottom left of your last picture do?
The gear selector on mine is fibre, as is the top gear on the layshaft. If your selector had bronze shoes I would guess your is older, assuming that machine development means reducing costs!
There are two dogs 180 deg. apart, giving two contact surfaces. Due to the dimensions of the dogs there is about 90 deg. backlash.
Personally I would have made them a tighter fit to virtually eliminate the backlash and thus reduce the hammering that the driven dog has obviously received.