Mystery Stoneware Teapot, IM or JM mark? Iain Markham?

This is a very nice and very heavy stoneware teapot not too big about 2-3 cup size.
It has 2 incised marks, one being a square with a diagonal line and the other a  W or M with a vertical line or perhaps an I
or perhaps not letters at all.
I would be grateful if anyone can ID this or has any suggestions.
Many thanks and I hope the images come out the right way up this time!
Here goes!

Maybe an early Iain Markham piece?
The square mark might be for the firing (cones), so possibly a student piece.

https://www.20thcenturyforum.com/t23341-iain-markham-cumbria