The ones made by Tram look slightly different than the Motorola or Maxrad ones. The center contact is slightly higher than the Motorola ones, and it surrounded by a plastic insulator about 3/8 inch or so in diameter. In some cases, they're passible, but don't properly mate up with the older coils. You save money buying Tram products, but it still isn't as good as the real thing.
Tram also makes one to fit a 3/8 inch hole, and they aren't worth the effort. Look here: http://shop.ebay.com/?_from=R40&_trksid=p5197.m570.l1313&_nkw=NMO+connector&_sacat=See-All-Categories
Which item did you get?