51/64" router guide bushing: what for?

Here's an odd one. My Woodcraft brass guide bushing set has a 51/64" OD, 5/8" ID size. This size seems to be included in just about every set, based on what I see online, but I cannot find a definitive reference describing how one would use that size. Is it for a hinge mortise jig? Someone here has to know.

I believe that bushing is in there for hinge mortising jigs and using 1/2" tool. Its the proper offset for the jig.

Agree with Mark. It's a hold over from the earliest years of guide bushing and router use on job sites. I was thinking maybe a stair tread template too, but can't say for sure.