Note that there are quite a few variants. The original 55 came out in 1939. There were 3 models- 55A (low impedence), 55B (mid impedence), and 55C (high impedence).I own a 55C which has been completely re-furbed by The Mic Doctor in Florida. They used 3-pin Amphenol cables. It was used by Franklin D. Roosevelt (remember- "A date that will live in infamy"?) and Douglas A. MacArthur's stirring speech before the D-Day invasion was done with a 55. Dubbed "Unidyne", it was the first unidirectional dynamic mic. Mine was used by Doris Day and Roy Rogers.
In 1947, the 3 models were replaced by one model, just called 55. It had a switch for low, middle, or high impedence. and the cool red silk inside the body was changed to blue silk. It was updated in 1951, and made quite a bit smaller and lighter, with the new Unidyne II capsule. Known as "the small Unidyne", that is what became known as the Elvis mic. It was used by every US president at least up through Kennedy. It was then discontinued for years, and reissued with an SM48 capsule, which, although perhaps a better mic, does *not* sound like a classic 55 at all.
Frankly, mine is a pretty cruddy mic, even if I give it the 100,000 (no shit) ohms it is looking for, but I use it for vintage radio plays, which I've been recording quite a bit of. What the hell? I had mine rebuilt because it is such a piece of history- a mic that changed the world.
If you come across an old one- note- The capsule was shock mounted with foam blocks which harden, and then crumble to dust over the years. The silk discolors and turns black also. The mic doctor really does great work, specializing in returning original 55's to spec, and has a bunch of new old stock amphenol cables and connectors. He is honest, and his prices are reasonable. Many of them were made, so it is not a particularly rare or expensive mic. It is worth it, however, to have it re-furbed. Note also that the original 55's don't quite fit the threads on most modern mic stands, although it comes close. You either need a vintage mic stand, or some teflon plumber's tape for the threads on the stand.-Richie