I Google Imaged this HP .61f of 1969:
http://www.btinternet.com/~wemxt03/start/engines/HP.htm
1969
HP entered the mainstream model engine market, they did so with a revolutionary new Schnuerle ported rear bell valve 10cc engine designed by Austrian Paul Bugl, the engine was of course the HP61. This innovative rear induction power-house was shortly followed by a front induction version, the HP 61F.
In order to facilitate the rapid introduction of a front induction version of the HP61, a new crankshaft and front housing was bolted on to the rear of the existing crankcase where the rear bell valve had been. The part of the main casting where the crankshaft had been was removed. Lacking lugs to bolt a back-plate to, a thread was cut internally and a large plug type back-plate, as found on many small diesel engines, was screwed in. The only disadvantage was that the exhaust stub was now on the right side of the engine, it was on the left side on the rear induction version.