There's quite a good explanation of the 24T2 here: http://zenithcarb.co.uk/zen24/?___store=zenith. The section on the air jet includes, "In some instances, where the full value of the drilled hole is required for the ventilation of the main jet, an air jet is not used". This might explain the absence of an Air Cap 150.
As for spares, Gower and Lee (01923 247300) helped me out recently with bits for a 24T2.
POSTSCRIPT
Have a look also at www.villiersparts.co.uk, they have a page devoted to 24T2 spares.
As for spares, Gower and Lee (01923 247300) helped me out recently with bits for a 24T2.
POSTSCRIPT
Have a look also at www.villiersparts.co.uk, they have a page devoted to 24T2 spares.