On my 1976 copy, it is both width and height that are smaller, but it does look like it could have been cut down in size.

Trotter wrote:

On my 1976 copy, it is both width and height that are smaller, but it does look like it could have been cut down in size.

Yes. I think you are correct. Looking at the text alignment on spine - it does look like it has just been trimmed at the top.

A third state of the 1976 (I have two now which have different cover textures, but otherwise appear identical to each other -- both the smaller size than the prior two impressions). One came from Australia (the more textured one) and the other from NZ.

And this one is possibly a fourth state (but not my copy, despite coincidentally being photographed on a similar background) -- sold in NZ recently.

My 76 3rd is also fractionally shorter. Particularly the interval between the top of the spine and title.