James Wesley:
Just a short note on the S-250 information. The original writer made an error in assuming all of these are shielded. There are several manufacturers of the S-250 and models differ in not just shielded or non-shielded, but also the level of shielding. NSA shelters (not generally available) have the highest level.
Here is a link to one of the manufacturers. My point is that a buyer should investigate the National Stock Number (NSN) of the unit they are interested in and contact the manufacturer to confirm that a specific level of EMP/EMI shielding is installed, if any.
Best regards as always, – Bob S.