Early 66 (1966 model), HD1A: diaphragm lever and fuel valve without fork. Fuel valve brought down by gravity only. ¹⁾
Oct 1966 sheet (1967 model), HD1B: fork on diaphragm lever for positive opening and closing of fuel valve.
Sorted leaning out of mixture at high revs due to earlier design and vibrations (from Aug 10th, engine 66X***(*)10611)
Jan 1968 sheet, HD1BC or HD1C: added little ball valves in the plastic cover for accelerator pump circuit to improve action.
It does not rely anymore on just the little rubber valve flaps on the gasket.
Improved accelerator pump delivery. Also added economizer ball in main jet circuit for improved mileage The later version was HD1D.
Most Tillies out there, even if stamped with earlier numbers, possibly would have been modded to later specs if properly serviced at the dealers at the time.
These carbs will start the engine easier if they're flooded:
Attach a small nylon cable tie underneath carb from one of the cover screws.
You can then poke (tickle) the diaphragm up through the vent hole in the middle of the plastic cover to flood it. Or adapt a tickler from a GB Amal carb.

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HD1D from a 1971 XLCH:

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The Andrews Super 45 is basically a Tillotson bored out to 45mm with a 45mm butterfly in it. ²³⁾

AP-45-2:
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HD137B 548 (“C” version):
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