Hi cooky,
phase plug 'length', and driver internal horn are two different things.
4" diaphragm drivers such as 2440/41/45/46/50/51 have almost the same phase plug 'length', but 2452 has shorter one. I have no info about 243x types.
M2 horn has about 4cm long throat, so I believe can be used with JBL 1.5" drivers too (2447,2450sl/51/52)
regrads
ivica