That's a nice find! Only 5107 examples of the IIa Typ 150 were made because of the war. They came in three versions, with the 3,5 Ektar, 2,8 Xenon and 2.0 Xenon. They are rare, but not extremely
rare. It took me a decade to find and buy all three versions.
The 2,8 Xenon in these pre-war Retinas is of the "Super-Xenar" construction, a five element version with the front element spilt to allow for better corrections at this speed. It is sharper than the post-war 2,8 Xenar at larger openings. You can use the later, coated, Retina filters on this lens if you want.
The lens is like the five element Xenar on the left.