Hello everybody, I bought this Dallmeyer lens hoping it was an 2D, But it looks to be a quick acting portrait lens with specifications close to a 1B, f3, but with 3 3/4 inches back focus instead of 4 1/4 inches. serial number is in the 3400's so no help in the archive. diameter rear lens 1 1/2 inches, barrel 1 3/4 inches. Does anybody know if variations were common with these early lenses? In the picture it's mounted in card board on a speed graphic. Thanks.