allen covered this , so mostly saying the same thing in a different way

Are all rotors supposed to have an advance mark from the factory? I have a Sparx on my Triumph and it doesn't.
Also, don't T120's and A65's have different timing advance degrees?

a rotor can go on either way , but only one face is usually marked .
early rotors had no mark ... later ones had 1 mark ... and the last ones have 2 marks , 180° apart .
but these are not timing marks
they are .... Centerline of the rotor-keyway marks .

its the crankshaft and its corresponding rotor- keyway
that has ... a degree angle ... to the cranks TDC

the rotor is a generic part that can transfer the ...crankshaft keyway slot .. to a readable line
on the face of the rotor . .. the same rotor will work with various timings of a BSA , Triumph or Norton

The primary cover holds the last part of the timing information .
with no cover ... the rotor mark has no reference .