According to my copy of the RCTS book "Coaching stock of British Railways 1978" the allocations of the Western Region class 43 were split equally between OO and PM using the class 253 set numbers - odd 253's to PM and evens to OO. The book shows allocations of OO rather than OC. There was one exception - 253027 was allocated to OO rather than PM. In practice the class 43 did not stay permanently with the trailers and soon moved between sets sometimes on a daily basis! I can remember seeing the same 253 set number on 2 adjacent trains at Paddington. I then realised that it was no good recording the 253 number and to record the 43xxx number!
No allocations are shown for the class 254 East Coast sets but see the stop press below
So we got PM allocations of:
43002/003/006/007/010/011/014/015/018/019/022/023/026/027/030/031/034/035
43038/039/042/043/046/047/050/051 and spare 43120
everything else was allocated to OO including the other spare 43121.
Hope this helps - it seemed so simple when I first looked!
Stop press - I just found more information in the back of the book.
The first Eastern Region allocations were:
43060/061 HT
43066/067 BN
43070/074/075 NL
The Eastern used Heaton at first for commissioning of the sets. They were normally commissioned to 'HT' before moving to other depots, often after only a few weeks.
First allocations were:
43056-093 HT
43094/095 EC
43096-099 HT
43100-119 NL
Then follow five spare power cars, although 43120/121 were delivered out of sequence, between 43061 and 43062.
43120 PM
43121 OO
43122 NL
43123 BN
43124 PM
The second batch of Western vehicles carried on the alternating pattern of the first 27 sets:
43125/126/129/130 etc to Old Oak
43127/128/131/132 etc to SPM
And so on up to 43149/150 to Old Oak.
43151/152 to Old Oak
43153-163 went to Neville Hill (163 intended for the NE/SW pool)
43164-180 to Laira
43181/182 to Neville Hill (for around 6 months, then LA)
43183-198 to Laira (of which 43193-198 very quickly moved to NL)