When the 365s were first introduced in kent, did they have their pantograph fitted?
The old system the SR used was doomed anyway- by extrapolation, units introduced 2000 or later should perhaps have been 47x (50s were 41x, early 60s 42x/late 60s 43x up to early 90s Networkers at 46x) but the 375, 377 and 450 are all four-car outer-suburban units- so they'd all have been, by that classification, 473.
The numbering system is now a mess- it perhaps needs a reset, with a DC, AC and DV range for EMUs and (3xx, 4xx and 5xx) with perhaps the second digit denoting manufacturers? (could work with DMUs too, obviously 1xx, 2xx and perhaps 3xx for electro-diesel hybrids)