1937 S/42 t block

[Chris from Minnesota]

I have a question for the Mauser experts or anyone that has experience with late 1937 1st variation Lugers. I understand that the 1st generation 37's were rust blue, I have a t block that has a rust blued barrel but the receiver and frame appear to be salt blued. The receiver and frame do not appear to be refinished, the tool marks and finish look very similar to my 1938 S/42. All numbers are matching including the grip panels.

My question is, during the transition period did they mix rust blue parts with salt blue parts? Has any one else see any late 1st variation 1937 S/42 with mixed rust and salt blue parts?