Holcomb,
Are you after a tachometer! I think the pickins will be slim if you are still running a 6 volt system. Stewart-Warner would probably be the oldest name to search for, but they wouldn't be in the '51 time frame or have the appearance of your gauges. These tach's were aftermarket items and weren't designed to match anything. If you are 'e-bay wise', that would be a great spot to get a feel for price and availability of a older tachometer. Good luck.
Stuart
