Yes I can see that Now Pete. Obv's an L1 'Series' Blank. A No.5 would have a row of holes down the central spine of course! ;)
Printable View
The central spine is narrower and the shape at the front and rear of said spine is different too. Made to take the steel grips. Early trials with this type of bayonet used wood grips that didn't last long as they were shredded by the blast through the flash eliminator flutes