This is a topic which has never really been satisfactorily decoded. As we all know there is a logic to A.C. car number prefixes - RS, RSX, CS, FEF, AE and so on, but the equivalent engine prefixes have remained unclear: If we have a UMB and a UMC was there a UMA ? As we then find UBS - the 'S' seemingly for Sports it implies that the basic version of the engine was a 'UM' so what did that stand for ? We then move to a 'CL' which becomes CLB, no CLA ? Then to CLBN, in which instance we know that the 'N' was for Nitride. We know the differences between the UM series and the CL series - white metal and thin wall bearings respectively for instance, but can these differences be made to match the letter codes ?
Seems strange that such a basic piece of information is unrecorded in the companies paperwork ? Perhaps it is somewhere ?