It's the screening mesh that makes a flame arestor, and it's the darned stainless steel that gets a USCG approval. I have a K&N Filtercharger on my Taylor but, since the manifold change I made is taller, I had to buy the off-road non-marine one with the drooped base. Their approved ones are all flat-based. As is I had to build my hood with a 2" arch in it (I was not cutting out the hood or building a "dog-house"), the marine one would have been 1 5/8" taller yet. The Admiral would not approve & I'd be in the "dog house". Maybe one day I'll buy the SS lid with the USCG stamp on it and fudge my way but for now, I am illegal with darned good intentions (and a paranoid search for fuel leaks).
Maybe Bowtie Rick can answer better on this one, but basically, there is no difference between the element in the regular K&N filters and the one in the "USCG" approved arrestors. The difference is in the housing... as you stated, one has USCG approval, the other doesn't. I can find you plenty of USCG approved arrestors that are not stainless. If you have a K&N element in a top/bottom configuration that has no additional holes etc; I'm pretty sure it would pass USCG teting just the same as one of the units already approved. The issue comes down to intended use and insurabilty.
Chris