Ottawa Mule Team mechanical differences
The tractors started with a poorly designed, 2 piece bolt-together front wheel.
Around serial # MT4097 they changed to a steel rim with a square sheet welded in the center.
The tractors started with a cast front axle.
Around serial # MT4113 they changed to a tube front axle.
All tractors had 16" stamped steel rear wheels,
except the Model 17 tractor which had 24" flat spoke type rear wheels.
Not all tractors had hoods, it was a $16.95 option.
Some tractors had the steel Mighty Mite hoods
Most had the round "Ottawa" hood.
Of the tractors with the round "Ottawa" hood, some were steel, some were aluminum
and some were a steel/aluminum mix
The earliest serial number I know of is MT4005 and the latest is MT4241
Engine Differences
I've only seen one Model 15 tractor with a Model ZZ Briggs.
Briggs stopped making the 8 HP ZZ in 1949.
If you have one your serial numbers should fall in this range.
1948 = 326810 to 361074
1949 = 361075 to 361103
If your serial number is lower than either of these, it's likely not the original engine.
The Model 15 tractor shipped with a Model 23 Briggs Engine
Briggs made the Model 23 from 1949 - 1957
If you have a Model 23 your serial number should fall in this range.
1949 = 0 to 27353
1950 = 27354 to 57104
1951 = 57105 to 110998
Since Mule Team production ended in July 1951, I didn't include 1952 numbers.
if your serial number is higher than these numbers your engine is likely not the original engine.
The Model 15A tractor could be had with the Wisconsin AEN or AENS
The difference being stellite exhaust valves in the AENS
The AEN was produced from 1948 - 1962
The AENS was produced from 1949 - 1966
Both engine are rated at 8.2HP @ 3600 RPM
I've never been able to date a Wisconsin engine by serial number, if you know how, let me know.