Solid oak convertible typewriter desk - M.L. Himmel & Son Baltimore

I got this solid oak convertible typewriter desk from a friend about 20 years ago. He told me it was valuable and to make sure I got it appraised if I ever wanted to part with it. The finish is not in great shape, but it is a solid piece and everything works. 

Three drawers with a locking mechanism (need to open the top drawer to release the lower two; I don't have the key). 

Pull-out writing desk above drawers.

Convertible top to reveal typewriter.

Metal label on front says M.L. Himmel & Son Commerical Furniture Baltimore.

I have three more of the brass leg covers that slide over the feet; these are about 2" tall sleeves. One is stuck on the front right leg of the desk; the other three are just in a drawer somewhere.

Any idea what something like this is worth? Thank you in advance!