Hello John

There are several external differences the 12-20 has:-

1. Sliding gate three forward speed gearbox
2.Steering box on the outside of U frame with shaft to cast iron steering wheel
3.Handbrake wheel on top shaft of gearbox
4. Water pump on front of timing cover driven from magneto shaft
5. Kingston governor with early style top cover and open centre shaft
6. Radiator flip over water fill top
7. Screw on front badge on radiator both Wallis and M-H, also some cast into top
8. Early Wallis 12-20 had "Wallis" cast into engine block, later allumimium plates fitted.
9. Flat spoke front steel wheel spokes
10. Starting handle with flared ferrule on the end
11. Throttle, advance and heat control rods had no knobs on the ends at drivers end
12. Engine speed butterfly in vaporisor not carburetor
13. Early 12-20 belt pulleys cast iron, later fibre larger diameter than PA
14. Early 12-20 had no belt pulley brake wheel and linkage from clutch control
15.Smaller kerosene tank than PA
16. Different length hood to PA
17.Chaff screen different size to PA
18. Flat fan belt with early screw tensioner
19. Cast iron sump inspection covers, mounted on the inside of U frame, wing nuts out
20 Front axle H section, round radius rods with adjusting nuts.
21.Tool box on platform
22. Seat spring and mounting
23. Wider fan shroud
24. Flip over engine oil filler cap
25. Spark plugs 90 degrees to head
26. Smaller diameter oil filter housing under U frame

These are just a few observations and I am sure I have left plenty for others to chime in with, together with several internal differences.

Malcolm.