OK, this is what I *can* tell you. I own a 55C, a high impedence contemporary of the 51. We think mine was made in 1946, just before they introduced the variable impedence 55S. Later, it was made smaller, in the 50's, and became what was known as the "small Unidyne", which became known as the Elvis mic. The 55 was the first unidirectional dynamic mic. The 51 came in 2 versions, 51 and 51S, the only difference being that the 51S had an on-off switch. Although it was advertised for music and public address applications, it found its main market in houses of worship.
Like all Shure mics of the period, it had an Amphenol connector, so I hope yours comes with a cable. Note that the capsule was supported by foam blocks, which have generally turned to dust or rock by this time. The red screen you see is silk. It was red in the original 55 series, and changed to blue with 55S. Usually by this time, the silk has faded at the least, or even turned black from mildew, etc. This does affect the sound. The 51, called the Sonodyne, was an omnidirectional dynamic with low, middle, and high impedence settings.
My conclusion about recording with a 51 or a 55? You don't, because it is obsolete, and doesn't sound very good. I use mine for radio plays where we are trying to duplicate the sound of a bad live radio broadcast of the 30's. Think of the burning of the Hindenburg- "oh the humanity". Other recordings made with a 55- "A date that will live in infamy". I haven't
been able to find out what years the 51 was made, but here's a couple of useful links:
First, the spec sheet-
http://www.shure.com/stellent/groups/public/@gms_gmi_web_ug/documents/web_resource/us_pro_51_ug.pdf
Next, the Mic Doctor. He specializes in refurbishing these mics, and can replace the foam mounting blocks and the silk, and can provide you with new old stock amphenol cables or wire them up for you:
http://www.artgraham.net/micdoctor/index_090203.html
Even if it doesn't sound that good, it's worth saving the mic as a piece of history. A model T isn't a great car, but you wouldn't have it towed away, either. Best of luck.-Richie
