Sorry for the late response, Daniel! The best way to determine when an instrument was made is to go by the serial number usually on the 2nd valve of the instrument. I did find a picture of instrument made by Ernest Seltmann in a book available online that includes instruments at the Metropolitan Museum of Art. If you have any more questions, feel free to contact us at simonettitubacollection@gmail.com. Thanks!
V & E Simonetti Historic Tuba Collection 
I have a E flat tuba with the inscription: E. Seltmann, Phila, PA 93 13
How would I determine when it was made. from what I read, the company went out of business in 1906 when his wife died.
Sorry for the late response, Daniel! The best way to determine when an instrument was made is to go by the serial number usually on the 2nd valve of the instrument. I did find a picture of instrument made by Ernest Seltmann in a book available online that includes instruments at the Metropolitan Museum of Art. If you have any more questions, feel free to contact us at simonettitubacollection@gmail.com. Thanks!