This is a truly one of a kind instrument - Selmer Radio Improved Alto #20633. One of the last of this model before Selmer switched to producing the Balanced Action - this may in fact be the latest surviving example. I had the horn overhauled by a woodwind specialist at Flying Squirrel Music in Raleigh NC, they were very impressed by the horn and commented that it would serve well either for classical or jazz music. I'm including some great extras - a Protec case that fits the horn well, and your choice of an Otto Link #6 rubber mouthpiece or a vintage classical piece that came with the horn when I bought it from Europe. Check my feedback and bid with confidence - I've sold some great horns in eBay, including a Mark VI, solid silver series III, and Conn 6m viii.  Additional info courtesy of Selmer historian Douglas Pipher:   First … there are actually two Selmer Altos with this serial number. The other one is easily differentiated because it has the optional trill keys. So … yours is officially known in the Selmer archives as #20633 B Now here’s the funny part. Your horn was shipped out in June 1935 … but the other one (known as 20633A) wasn’t shipped until 1936. Your horn was originally silver plated. It was sold to a music store in Paris. You are correct that your horn is one of very last Radio Improved horns made. More correctly … it is one of the latest Radio Improved Altos made … if you omit the later ones which qualify under the title of “Late Radio Improved” horns. Basically, it seems to have worked like this. (and I am simplifying a little) April 1935 - earliest Balanced Action samples were sent out to various markets. June 1935 - Radio Improved production ends and Balanced Action production starts. 1936-1937: Additional Radio Improved Altos were completed and shipped. Some of these are with serial numbers earlier than yours and some are later. So, your horn is certainly one of the last handful of regular production Radio Improved sax built before production shifted over to the Balanced Action.