Air filter with an inlet of 39mm suitable for pre-65 trials and many Japanese applications as follows:-
BSA / Triumph etc. all 375 Monobloc Carbs. The inlet is slightly oversize, but will clamp down.
Honda XL125S/RC, CB125RS/T, CB550F/K, CB750K 69-76
Suzuki DR125S, GS550 through 79
Yamaha DT125, XT125
The filter is approximately 75mm at it's widest point tapering down to approximately 50mm. The overall length is 80mm from back to front. The end caps of the filter have a nice deep chrome finish.
These filters should always be oiled before use for maximum filtration efficiency. K&N Filter Oil is available from this site