Corm:
Re: TM-19609
Sheldon Machine Co., 4258 N. Know Avenue, Chicago, IL 60641
Book lists Tool Room Lathes and Sebastian Lathes. The Sebastian were built from 1936 thru 1946, the year the Sebastian Division was phased out. All listed Sebastian used a 6 digit serial #. Book notes that some Sebastian parts are still available (as of 1983 when this book was printed).
The Tool Room Lathes are listed as series "R". The first year of production is shown as 1945 w/ ser.# beginning w/ 0680; in 1952 the ser.# began w/ 11008, in 1955 began ser.#16955. Serial # 22568 started the production for 1956. In 1962 the serial #'s began with 25700.
Note: All listed Sheldon & Sebastian serial numbers do not show any letter prefix or suffix.
Corm, it appears your thinking mid 50's is very likely with your 5 digit 19609, which falls into the 1955 model year group.