Box says Centralab Blue Shaft Radiohms Type B- 15/16" Controls Fastach Dual  F1-15 Front 2500 OHMS Taper C5.  I don't see anything blue here but the part does say F1-15 on it.

Can't tell you anymore. Got left these by my father when he died and I am liquidating his stuff. He was a ham enthusiast and fixer upper buyer/seller. Me an offer. Just want to pass on to anyone who can use them. Anything I can do to check them out let me know.  I have limited tools: a multimeter if that helps. All items "as is".