Thanks for that picture, the O&R branding should date the engine to Nov/Dec 1974 or earlier.
There are two possibilities for the Chicken Power engine, with the threaded exhaust manifold, remote mounted fuel/gas tank and a silver painted /starter blower housing with toggle switch (the numbers in brackets are the release date from the O&R list);
Type 13B-340 (7-1-74), this had a flexible exhaust pipe.
Type 13B-406, this has a tube exhaust pipe.
I'll have a closer look at the part number differences another day (the carb parts differ between the above two engine types).
Other engines that it can't be are;
Type 13A-181, older style engine with round starter mech.
Type 13B-328 (8-29-73), has five cup exhaust muffler.
Type 13B-341 (1-21-74), has five cup exhaust muffler, but could also be supplied with threaded adaptor.
David