Submitted by Patrick
PLEASE: If you are in the military, let go of the SDU-5/E and get a Firefly Two (NSN 6230-01-448-8340 w/pouch; 6230-01-411-8535 w/o). Keeping the SDU-5/E going is creating a problem in acquiring the replacement BA-5374/U battery, which has no other use. The cost to DoD for acquiring this specialty battery is very high and will continue to rise.
ED - Patrick. I agree, finding replacement batteries for the SDU-5/E is getting harder and more expensive, for both civilian and military alike. If you're going to continue using the 5/E my recommendation is you go to an outfit like PRC68 and get an adapter cap which will allow you to run the strobe with a pair of CR123 batteries.
Baring that, you ought to upgrade to a strobe that runs off of AAs or CR123, of which there are many on the market. The ARC Firefly2 (I think I found the proper civilian reference) you suggest is a white light only strobe. While this makes it a fine piece of kit for civilian search and rescue operations, it lacks a IR filter for use in tactical situations (which is why I'm a big fan of the ARC MS-2000(M).) Do the Gov. issue ones you mention come with an IR filter?