I'm 99.9% sure that the one for sale on ebay will fit a Colt, and provide the correct timing advance, although it might be of a different type of construction to the one on your colt - so parts would not be interchangeable between the two.
The seller obviously hasnt been able to take the advance retard unit off the cam, which is why he's selling it all as one lump. you need a certain size threaded bar to take the unit off.
The good news is that the cam is the later ramped type which is the better one to use.
Sometimes i buy things on spec and if they dont fit i sell them on again, the only loss is the postage.
Square 4 mk1, Square 4 mk2, Ariel Colt, Matchless G12, BSA C12.