[Lynn Eades]

Luigi
it would help if you could identify the carrier you wish to follow.
FYI,
A Mk I is British built.
A Mk I* is Canadian built. This is across the board, firearms as well. (the * means Canadian made)
The next thing to consider is that the transition from MkI to MkII in a Mortar carrier does not coincide with the transition from MkI to MkII in a Universal Carrier.
The transition is usually as a result of a major re think of the stowage (or less often a slight redesign of the hull)
From Nigel Watsons Books.
The first three contracts for Mortar carriers do not identify the mark, and I make the assumption that the transition happened from MkI to MkII at T82618.
I have a plate off Mortar Carrier T80908 It has no Mk on the plate it merely states,
"CARRIER,3INCH MORTAR W.D.No. T80908" I assume this and any earlier Mortar carriers to be retrospectively MkIs.
This transition in British Universal carriers appeared to happen at about T200,000 (100,000 carriers later)
The carriers you show in your photos are likely much later and in a range T 250,000 or say 1944 as opposed to 1942 era.

So back to your question. The early MkII Mortar Carrier had the same height adjustable seat in both sides of the carrier as the early MkI universal.
At what point in history the fixed gunner's seat happened, I do not know.
I hope that helps.
Anyone with info to the contrary please speak up.