These are the knee grips and a metal mounting plate (below) as fitted to the petrol tanks of the sportier motorcycles in the 1920s and 1930s; they were designed by a mr George Dance who was one of Sunbeam's most successful racers. The label on the mounting plate explains it all:
RACING KNEE GRIPS
Racing Knee Grips, if properly and carefully fitted, will give you that armchair feeling at any speed. By drilling other holes in this plate, a variety of angles and positions can be obtained after the studs have been sweated on the tank.
They were designed to be fitted in conjunction with the saddle and footrests and as TT winners will testify, perfect control of the machine is established if the correct position is obtained.
Can't do without them! Importantly, an original and barely used knee grip mounting plate has resurfaced and completely unexpected, it was painted red in the factory! The paint will be analyzed to establish the right shade of red and John Bull labels will be printed and made available to those wishing to make the restoration of their motorcycles absolutely perfect. (picture contributed by Alf!)