I am in Europe and haven't seen this exhibition. But I have the book, which I like a lot ! Some of the early work is really beautiful, and new to me.
I also have the first edition of her 1972 Aperture Monograph (yes, with the raincoats girls) and just had another look to confirm what I already thought: the fact is that she sometimes has black borders on top and bottom, but not left and right. Or she has black borders left and right and not top and bottom. Then, in both cases, one black border can be much thinner or even absent because she was not very precise when placing the negatives.
The reason is simple, she used an enlarger with a negative holder that was very tight and just did not include full black borders. The early Meopta enlargers were like that. Maybe she used a US made equivalent. The reason why sometimes the borders show at the top and bottom, and sometimes left and right, is because she either put the neg strip in from left to right, or from front to back. 6X6 negatives often do not have the exact same size for top/bottom versus left/right. Or, it can also be so that she used different cameras. Negatives sizes are hardly ever precisely the same between cameras.
Anyway, I don't think her estate would accept mats that cover parts of the image. In her case the black borders belong to the image.