Definitely 126....I've just checked and I have some similar transparencies and negs in front of me here, which match in all respects. The white edges were part of a pre-exposed "frame" around all the shots on the film...I've read that this was done to avoid large areas of unexposed film when it came to processing, as this could exhaust the various chemical baths at different rates. Maybe PE or someone with lab experience could confirm?
I'm sure that the slight oversize of the image compared with the nominal 26x26mm is to allow for small variations in registration when the negs are printed, or the transparencies put into slide mounts.
Agfa Rapid film used standard 35mm perfs, with the film being wound from one special cassette/cartridge to the other (I think that I still have a cheapy Rapid camera somewhere in the attic!). It was a much less popular format than the Instamatic 126, but was arguably better in using a pressure plate and standard 35mm perfs, and also a square image.