Over the years, I have ordered three replacement batteries for my Regency MX-4200. The first one was from a local radio dealer that was able to special order the battery from California. Sadly, that company has since gone out of business. The next one came from eBay. Finally, my last one I purchased was from
Scanner World, USA.
The battery pack usually had part number MA529 on it. However, the battery from eBay that came from Japan had part number: JBP4NS, and was recommended by AOR. With that said, a friend of mine ordered a battery pack for his MX-4200 over in the UK around four years ago, from
Strikalite. The good news is that Strikalite will custom build you a battery pack for your MX-4200 from scratch.
73's
Ron