Yeah, and I thought I had a match in a 37mm, but the rim is different. Some of the pics are really unclear (distorted scans, it appears), and trying to match a shell without exact dimensions, and without listed dimensions is difficult. I figured the volume of AA rounds fired would be much higher, and it would be the most common casing, but that's not necessarily a good assumption. The proportions of the neck are similar to other 37mm rounds, but again, the rims are entirely different, at least compared to what I've seen so far. Still looking- there's an awful lot of information to dig through.