I have 2 gold face tachs. One is identical to the one in my BT7. The other appears identical at first glance but has 2 differences. The number on the face is RN. 2301/01 vs the BT7 RN. 2351/03. A more obvious difference is the redline. 5250 RPM on the BT7 and 4800 RPM on the other one. Any ideas what the application is (was)? Marv J