Thank you for contacting Mearto. This is a Penzel Mueller slide trombone. A New York-based company, it was founded 1881 by German immigrants and ceased production in 1970.
Penzel Mueller woodwinds are generally less expensive than others, but they’re also rare. Have you checked the condition, as far as whether it’s playable? Visual signs of age are pretty standard on most instruments from this era. I have not found anything with the name Amborg, but I assume it is the individual maker. There was nothing I could find to indicate it is the name of a line or model. I also wasn’t able to find info on the serial number unfortunately.
Penzel Mueller woodwinds are generally less expensive than other brands, but they’re also rare. The Fair Market Value for these instruments go for around $100 to $250 at auction.