The O&R 60 small port engines were produced from 1939 to about 1947 or so.  At that time, the design was changed to large-port and some front rotary valve engines .The factory in Los Angeles built a very limited number of the 23 engines during the war, using parts on hand, but no 60 engines, that I know of.

Your engine has a relatively low serial number, indicating it is pre-war, or 1941.  The Model Engine Collector's Journal for January 2011 lists serial numbers of 55 examples of the 60 "special" (small port) engines.  Serial numbers range from 1125 up to 078023.

I believe MECA has a more complete list somewhere.  You might try contacting MECA for more information.

The four O&R 60 "special" engines I own are 2165, 8949, 17832 and 045351.  I believe all post-war engines had a "0" serial prefix.

Floyd