My memories of the Eureka/Rebecca in both the JP 3/4 and the Mk1 Andover was that it was almost completely useless. The Andover display was CRT as opposed to the dial in the JP which seemed to roam the miles scale on the dial randomly.
The Zero Reader was also fitted to all Mks of Andover I flew and was simply a poor man's flight director but worked ok as long as the I/B course was set with the drift applied.
Happy days - when every ILS was a raw data ILS.
mcdhu