[Pete360]

I own a 38 tenor and a 39 alto. The big mechanical difference between the 38 and 39 is in the octave thumb key. The 39s have the modern Selmer style thumb whereas the 38s have one more similar to a Balanced Action. Also they have different pinky clusters. The 39 will have one metal roller on it.

 

[Pete360]

Yours looks to be a 38 but can't tell with out seeing the pinky cluster. If it has one more similar to the old Keilwerths then it would be neither a 38 or 39 but the older 37

 

[Pete360]

Both are 38s