Hello Ian,
according to "Douglas the motorcycle range" compiled by Douglas Frost, there are three types, 1924 had the normal TS type, 1925 had a zoom zoom type and 1926 had the first half of a TS with a 1 1/2" pipe out under the rear chain and stops just after the rear axle ending in a small fishtail. I could send a rough plan of this as I am struggling through one at the moment, regards Jim