Aye Up MikeLA,
I've just re-read your OP and may have misunderstood it.
I've just tried the Austrian stainless mug on the Pathfinder bottle and although it sits fairly high it
sort of 'locates'.
(The Austrian mug fits near perfect on the Brit '58 pattern bottle and is closer in profile to the Crusader mug than that of the Pathfinder mug).
If the Crusader mug (which fits the Brit '58 pattern bottle)
has a slightly larger mouth area it might just fit the Pathfinder bottle.
I might be able to get my hands on a Mk1 mug later this week to try it if you can wait out.
Alternatively, I reckon that if the Austrian mug were heated up, forced onto the (full) Pathfinder bottle and then allowed to cool off it might adjust to fit - maybe the same with the Crusader.
I understand your rationale re multi-purpose/function, I'm that way myself. On that subject -
FYI - The handle of the Pathfinder mug
is long when compared to '44/'58/Crusader.
Like the Austrian mug it is intended for scooping and for use in an open fire bed (the pathfinder mug already has a knife/bayonet slot in the handle - the Austrian mug is easily adapted with a couple of wire loops for knife or better still a suitable stick). Perhaps not so easy with the wire type handles of other mugs.
I prefer the Austrian mug over the Pathfinder, the latter which I think has a narrower profile similar to the US '44 pattern mug and not as good for boil-in-bag meals.
I have used the Pathfinder bottle in a fire bed on one occasion to see how long it would take to boil (best part of 30 minutes) but after someone commented that the bottle is made in two parts and has a seam (I think it was on BCUK) I thought that it might be something not best practiced on a frequent basis.