I just saw this posting, while searching for sonic info on silver mica caps. Assuming the cap still works, it is a just sliver mica cap, I doubt it is as good as modern ones at its working RF frequencies. But if you need the value, and you have it, then you can give it a try. It should work as well as any other cap in that value range.
The catch is, like I said above, it is old and most likely has more losses than modern SM caps at that capacitance and voltage range. They still make them today for contract military jobs, where this part is specified, and it would be far too expensive for a redesign.
SM caps are usually pretty small, in the region of pFd and hundreds of pFd. They excel at passing RF frequencies, so their losses are very small in the RF range. Case in point, I had a home brew FET input line amplifier that would oscillate when the chassis cover was off and work perfectly when the cover was on. I had a basic ceramic cap for RF purposes on the input with one inch of wire on each leg between the cap and the FET input, It wasn't an NPO/COG or even a X7R, but a more lossy one. Switching to a SM cap right on the FET input killed the oscillation completely and it was rock solid stable afterwards. Granted the two inches of wire I deleted helped, but you see my point. There is a reason they are a lot more expensive than ceramics, even good ceramics.