Speedo Running ''Slow''

['68OrangeSunshine]

So I still have the original speedometer in my '68 C/10 Stepside, a truck I bought at 23, and now I'm 75. It's got 407,000 showing on the odometer. [I bought it at about 80,000. It was a 5 year old truck.]
Just lately it's been indicating ~40-45 MPH when GPS says I'm doing 50-55.
I checked the cable connections: Underdash was secure. The cable to the correction box was secure. Corrector was secure, with a little swing to it -- which I think is normal.
Next step will be to pull the little speed corrector box -- maybe it's turned to jelly inside after a number of years -- IDK.
A few years ago, I pulled the inside speedo cable out and dusted it with graphite powder as I inserted it back in. That took out the jumpiness. Not jumping now, but it won't go over 40 or 50 indicated.
Long time ago, because I went to taller tires, it consistantly read slow. I took it to a shop. They mounted me on a dyno, and they called out what speed my wheels were turning, and I noted what number was indicated. I made a Correction Scale and it's still dymo-taped to my dash above the ash tray.
If, after I pull the Correction Box, I get the old correction numbers, then we'll know that's what it is. Otherwise it's in the clockwork of the OEM Speedo -- and after only 407,000 miles. Shocking!