What do you mean "they never progressed" Do you mean the Brits?
What you maybe dont follow is that the different types evolved at different times. As with the Brens, there was a MkI and then a MkII. This was before the MkI U.C.
The Armoured O.P. evolved rather quickly. The Mk I, followed by MkII (see Nigel Wards) and by 1942 The AOP MkII was obsolete. A directive was issued to withdraw them from service around april that year)
I suspect the change from MkI to MkII in Mortar carriers was also at a different time to the other U.C's.
My Armoured O.P MkIIIW was built in late 1941 at Dagenham, and so we are talking about Mk III AOP's, almost before any MkII U.C's were built.
Yes the Armoured O.P MkIIIw is a similar layout to a MkI U.C. in that the battery is located in the left side rear and the air exit holes in the rear plate of the lower hull are rectangular.
Do you have pictures of a differnet hole arrangement?