9x12 cm negative carrier dimensions

I have a 9x12 cm camera that I would like to use and have been gathering all of the necessary supplies. I got a great deal on one on eBay for a 9x12 cm Omega negative carrier. However, when I received it I was surprised to find that the actual dimensions of the opening are quite a bit smaller than 9x12 cm. The window is about 3 1/8 x 4 1/8" (while 9x12 cm is approximately 3 1/2 x 4 3/4"). My initial reaction was that this was actually a carrier for 3 1/4 x 4 1/4" film. I emailed several other people that have 9x12 cm carriers listed on eBay to confirm that theirs are actually 9x12 cm. I've been told that one is 3 5/8 x 4 5/8" and that the other is 3 1/4 x 4 1/4". The former of those two would not be supporting the film in one dimension if the dimensions given to me are correct. The latter has the same dimensions as the one I received. A piece of 9x12 cm film will fit in the carrier I received, but it crops about 1.5 cm off in the longer dimension. Obviously the opening needs to be smaller than the film itself, but that seems a bit extreme to me. Does anybody know what the proper dimensions are for a 9x12 cm negative carrier?