It's an interesting case, the early carby's, the book will indeed list the same carby, the 3F, but it variated extensivly, to start off, when it first came in (replacing the Villiers Carby) it was attached to the engine using a large brass compession bolt lock, similar in concept to the locking mechanisim on most pedistal fans, this was dubbed the "Brass Nut" carby, but is technically a 3F to my understanding.
The Brass Nut design was a flop, simply, it came loose during operation, so the lock was changed to a simple clamp, such as is on yours, however, the cerclip cap lock remained, untill around the model 4 time, upon when the carby was altered to acept either a normal, spring clamp cap, or the two cable spring clamp cap needed for the Predicta without needing to produce two different carby bodies to accomodate the predicta.
As the predicta failed, plugs were released go fill the second throttle cable hole when converting them to standard throttle, abd the original casts for the two hole Predica capa were retained, but the second hole was just left undrilled, leading to 3 caps for the post-circlip design 3F carby's, one hole, two hole (or one plugged one open) or space for two -one hole.
Then cane the 3L, same as the 3F, minor jet changes, but mainly just sized for a lager intake port, and later the 3S, same design, shape and appearence as the 3L (&3F) but jetted for 160cc, and marked to distinguish it from the 3L, around the same time the 3Q & G2 designs were released , somewhat experimentally, but the popularity of governing on the G2 lead to the G3 taking over for almost all applications shortly after.
