Joe has it right about the widow. She's the one who decided to discontinue making the gauges. Brownell's no longer has any.

I was measuring a 20ga bore at a Cabela's Gun Library. The head got stuck. When I pulled the gauge out, I had a shaft and no head. When I pushed out the lodged head, it fell to pieces (spring, little ball bearings, etc). My local gunsmith looked at it and determined that was a ship he couldn't put back in the bottle. But very good service from Skeet's!