Strictly speaking there is no such thing as a Series B & Series C RFM.
What everybody calls a series B RFM, is the first "early" type used on series B machines. The second type is sometimes called "universal straight" which came about due to the introduction of the "Meteor". As the singles have the rear drive on the opposite side to the twins, the earlier RFM could not be used on a single due to the rear brake cable abutment getting in the way of the chainguard. This was changed to the later type that is the same on both sides with a removable brake cable abutment, hence universal. At the same time the factory took the oppotunity to lengthen the RFM slightly. This due to the jig design moved the seat pivot bracket on the RFM backwards slightly and so altered the geometry of the rear seat stays. Not many RFM's were made like this before the error was spotted, and so they corrected it with the curved seat stay brackets to restore the original seat stay geometry angles. Sometimes called "universal curved", this is what most call the series C RFM, except some series B machines were built with such an RFM as well.
So if you see what you think is a series C RFM with series B frame number, don't assume its wrong. Ask somebody who may know like the registrar.
Simon