Here's the reasoning process.  The site lists a dealer price for a True Tone alto at $600.  That price does not differentiate between the later True Tones with improved keywork and earlier ones with the more primitive keywork such as yours.  The serial number places the date of manufacture in 1923.  The lack of  a front F key on your horn limits its upper range to high F and deprives the player of some important alternate fingerings for high E and F - a major drawback for anyone wishing to play the top of the horn.  The left hand pinky cluster is extremely primitive with drawbacks to ergonomics and functionality that Buescher would mitigate with redesigns within a couple of years - another strike against its value.  One important question is whether  the F bridge arm (operated by depressing the right hand stack keys) can close the G# key while the G# keytouch (pearl button on the LH cluster) is depressed.  If it can't, that would be another major drawback that would decrease its value.  So when you adjust the dealer price down to reflect a private sale price, then knock some more off because of the functional drawbacks of the keywork on that particular horn, you're left with a few hundred at best.  It's a beautiful looking horn, but more of a beautiful antique than an instrument that someone serious about playing would be interested in.