There are not too many lawnmowers from this vintage that used a twin-cylinder engine. The brochure said that Norman twins were used on the larger, 24", 30" & 36" machines in the 1950s.
These engines were manufactured by Norman Engineering Company of Upper Grove Street, Leamington Spa, United Kingdom. A Grace's Guide reference may be found HERE.
I do not know whether the Greens machines only used the one model Norman; but I do know that the 24" used a T300 model. These would have been sweet-running beasts! [Photos by Graeme Brown, Brisbane]