This is the Erskine JAP as used by Soren Sjosten and which has undergone extensive restoration.
The frame is of Erskine design who also tuned the engine which is the 4B JAP motor.
Both the engine casing and head are of light alloy with a Wellworthy Cylinder and had a 2V head.
It also housed open hairpin valve springs and a Wal Phillips fuel injector.
N.B. One or more images are images are missing (×).