Linhof Technika IV back

I recently purchased an early Technika IV (s/n 63089). I noticed that
with my longest lens (210 mm) the corners of the image were getting
rounded off on the negative. I took a closer look at the camera, and
sure enough, the cutout in the camera back behind which the film sits
has rounded corners. If I put my film holder (I use Fidelity Elite
holders) in the back and look through the front standard at it, I can
see that the rounded corners do indeed impinge on the film image area.
I've never heard of this 'feature' of the Technika camera.

So, my questions. Are these rounded corners on all Technika IV's? The
back on this one looks unmodified, so I assume it's as originally
manufactured. And what do people do about this problem (other than just
accepting it)? It looks like it would be very difficult to file the
rounded corners out without removing the bellows first - and the
bellows appear to be glued on, so I'm reluctant to attempt that.

Thanks - Rob