Most enlargers have some falloff around the edges, I used to have a round piece of card on a thin stick, which I used to hold back the centre when exposing films. This, like most things, to do with photography, was less marked the more expensive the kit. The problem stemmed from having a bulb as a light source.
When stepping up to doing COLOUR prints from slide using cibachrome technology (the temperature was less critical, print from colour negative was a lot more sensitive to temperature variation) this introduced a whole vista of added complications, in terms of setting the filters and of course correcting the exposure. Added to that, I would assume that all the prints were done by one individual, by hand, which means that there is that persons interpretation of the print. It may be that he liked framing the images slightly and deliberately held back the edges. There is a whole world of unknowns, in the way the printing was done. Something I have droned on at length to "so called" photo experts saying that these images, are somehow indicative of some form of fakery.
As an example, the image below took me over 1 hour of individually exposing various areas and holding back others, I had to cut out templates in the end to get it right. Without adding "my interpretation" to the print, the sky would have been totally grey, the buildings in the distance would have been under exposed, you would not have seen the fine wire lines in the sculpture, etc. My main point is, this was done around 1978/80 using technology of the time so in terms of relevance, it is totally relevant.
For anyone interested the extra graining effect was achieved by using a Canon F1 camera, Ilford HP5 film and pushing it to 1600 which makes for a more sharper image but with more graining.
