When these plow planes were being used by makers ( 19th, 18th Cent), the 
size of the plow did not need to be very accurate.  The inserted piece 
was (hand) planed to fit whatever size slot was cut....

Before the Industrial Revolution, the "ruler" used was as circumspect as 
the iron size.  But then came the theory and practice of standards..... 
and availability of commercially made, standardized rulers.

It is only in the late 19th and 20th/21st  centuries that we cared, or 
could or would measure for absolute accuracy.

Now we can measure to 3 decimal places for about $20.

And we (hand) plane to fit.

Progress.

Frank Filippone
BMWRed735i@g...