Type B reverse on Proof Washington quarters?

sinin1

When did they start using these?

I have seen written in 1936 and also in 1937 -

are 1936 proof reverses type A or type B or a combination?
did the switch occur during the 1936 year so there are multiple type reverses?

ProofArtworkonCircs

As far as I know all 1936 proof quarters are type A.
All 1937 proof quarters are type B.
The only year with proof switches is 1968 S.