I think you must be looking at an M41D. The J gear is quite different from the gear in the original M40.

Generally sedans use yellow and wagons use green. The gear in the M41J is black and differs in size and shape. I checked when the M41J went into my 67 wagon and did not think it would fit.

I kinda wish I has not already modified the 1800e's rear to fit into my wagon. It'd be a bolt-on replacement for the 68 sedan and I'd know that the gear and rear were matched perfectly.

A clarification...
I know that my commute is 15 miles each way - verified with mapquest and with the trip odometer (before that part quit). Until I switched gear boxes, the main odometer has been spot-on and I have no doubts about its old accuracy.

I calculated a 0.83/1 mile error by dividing the known commute distance into the distance shown on the odometer since I changed gearboxes.

What I'd like to know is if 0.83 is on par with the error that would be expected if you took a 15" tire/M40 gearbox/4.10 rear and replaced that with a 15" tire M41 gearbox/4.10 rear end. The M41 was geared for a 4.30 rear, not a 4.10.

So does the "missing" 0.20 account for the 0.83 odometer error mathematically?
I'm not sure how to run the equation here.

Thanks for all the responses so far.