PART NO.  SPL.PAGE  DESCRIPTION                                                            
AEH404     TC.A.12   Oil seal (2) -(cyl.head, valves, engine, Twin Cam)

This part is a rubber strip seal, item 30 in the illustration below right (click for larger image).
It is 5.5 mm thick, 9.7 mm wide and 140 mm long (0.219-in x 0.382-in x 5.51-in).
Thickness can be compressed about 0.050" when measuring with a caliper.
It seals the joint between front of cylinder head and the engine front plate. It sits in the curved valley where "21" is pointing in the illustration at left below (2 places). These seals are secured by seal retainers 31 and 32.


See also T.C. Engine tech article TC-338.