Was wondering since a lot of us are confined to flame arrestor size because of clearance issues if we could safely and legally use one of these to gain more air flow:

https://www.knfilters.com/universal/X-stream.htm

If not could somebody please explain what makes them different from the marine flame arrestor material. To me it appears to be the same stuff they use for cars.

I could be wrong but I am pretty sure that marine arrestors can withstand a backfire and not blow the filter media and contain all the associated flame within the filter. Automotive styles like the one you posted may blow out the filter media allowing a flame to enter the engine compartment. becides, if you have a fire and the arrestor isn't uscg approved no claim will get approved

marine arrestors are meant to contain fire/flame they do not filter the air ,
no reason to filter air when your on the water...
No dirt in the air.
I would not run without a flame arrestor,, in a closed compartment a backfire could end your day or your life..

You would need to replace the medium with metal medium like the screening used in a regular flame arrestor.

I understand that it has to be USCG approved. I'm currently running a k and n marine flame arrestor. My question is if we could safely and legally use the top cap from k and n on the marine flame arrestor. The k and n marine flame arrestor appears to use the same medium as the automotives do. But I may be wrong on this.

I actually just received a 14"x4" K&N Marine flame arrestor, and I cannot tell the difference between it and all the K&N's I've installed in my land vehicles. I honestly believe the only difference is the special marine SAE stamp on the element and the USCG approved stamp on the base. I'm fortunate enough to have a little room under my deck and the deck has a built in ram air intake system with dual NACA ducts on the sides. I will have to cut a little of the under side of the deck to clear the taller filter, but this now places the flame arrestor squarely inside the fresh air system. I found a custom velocity stack that will work with a 14" filter with a top that is shaped like the K&N air horn device to smooth air into the carb. Be careful when selecting an arrestor of any type, some of them don't flow much air. I was originally looking at the K&N 9" which comes on a velocity stack. I called their tech support and the 9" won't flow enough air for my 524 and will create enough vacuum to close my power valve.