Rev counters on late CSR twins

[Andre]

Bacon's restoration guide shows what looks like a matched pair of chronographic instruments. The white needle with a silver centre, grey middle to the black face model. I have several speedos of this pattern but have been unable to track down a rev counter to match. I have fitted an older style black faced rev counter and my local instrument repairer assures me that what Bacon shows is a magnetic counter. I am not convinced. Does anyone have any information?