Can't speak for the pre-68 speedos but my 73 has a pin for the needle to rest on at the 0-end of the scale---maybe yours fell off inside the cluster ?? As far as the speed being off you can test it by taking the car a measured mile on a highway. Travel at 60 mph for 1 minute---you should have covered 1 measured mile. If not the speedo gear needs to be changed in the tranny where the cable threads on---the adapter gear. Hope this helps.
Should have a STOP pin. Asume u'v got correct speedo gears in ur trans. If nu repo avail , propbably most reliable way to go. If u fix it ur-self , u may find lotsa need 4 ur talent. Old Speedos R much like old clocks.
I don't think that the early speedos have a pin. There's a stop inside the mechanism. So I'm thinking the problem is:
1) You think you're stopped, but you're actually going 15 in reverse
2) The stop inside the speedo is compromised
3) The indicating needle has been removed at one time and replaced in the wrong spot.
There are a number of map/traffic apps that now give you the speed you are traveling when you are driving down the road. You can compare your speedo with those. If it's consistently 15 mph slow, I'm thinking problem number 3.