Hi there

As has been said (and pretty redundant now that they have arrived) the difference in the base and backrest is only the difference between the regular ones and the ones for the sitting inside the adjustable frame. The ones for inside the frame being slightly smaller.
Not totally understanding what you have said - but the seats I purchased are the same size as I took out (only better as there new)

You did have me worried, been admiring them on the floor in the workshop for a couple of weeks (as been busy with work)........ but after reading your post there was a quick sprint to the shed to fit them and they not only di they look perfect they aslo fitted............perfect (you had me worried there Diana)

All the best

Wayne