[JB White]

Just for the sake of comparisons, I too have a B prefix '41 Lithgow with the cutoff slot and marked as a MkIII, not a MkIII*.
My '41 C prefix from the Orange production lacks the slot and it's marked as a MkIII*.
Until this thread I was always under the impression that 1941 was the transitional year in actual production.

Also, my '18 LSA is a MkIII and has a cutoff. I surely wouldn't mind having an '18 LSA marked MkIII* to keep it company.

 

[JB White]

Too bad we didn't live closer to one another, or we could make temporary custody arrangements.

I think you have the more desireable specimen of the two.