As a means of understanding the versions we have to start at the most recent.
This unit, the C3, is described as such shown on the Air Ministry drawing It is a '1942 style - high intensity fitting with a small seating ring'. It was installed on a large number of the 160 WWII stations equipped with the system. I am not convinced about the 'high intensity' as I believe it contains the same internals as the earlier fittings.
C3 Compact fitting
The C2 is also shown on the above drawing, and is described as the '1941, low intensity - large seating ring' fitting. This is the one found at Polebrook by PB.
C2 Star fitting
Now the confusion begins - what is the C1? The Polebrook thread highlighted the fact that this 'C2' fitting is not identical to the units at Silloth. Both are star-shaped, however The latter has a 'flat' at one point, whereas the C2s have two indented areas. I also suspect that the Silloth may have a larger seating ring than the Polebrook. I suggest we call the Silloth types the C1. (Silloth was the first Coastal Command station to be equipped with Beam Approach and Contact Lighting)
Silloth 'Star' Fitting
Which then leaves us with the Chance manufactured unit found on the pre-war airports (Manchester, Croydon and Heston). This unit (strangely similar in appearance to the C3) didn't have nearly enough light available for aircraft actually on the runway. I guess we call it the 'pre-war' fitting.
Chance fitting as used at Croydon c.1938
Action: We need to measure the lamp and seating ring diameters of the C3, Polebrook C2 and Silloth fittings to see if there is any correlation. If so, why was the unit redesigned? In over ten years research I have found virtually nothing in TNA on this subject, other than details of the pre-war unit.
Note: According to the AM chart the C2 and C3 were known as C2A and C3A whan the internal bi-directional beam concentrator prism was fitted, What about the earlier fitting?