Rev Counter Polarity

[MINSTER 407]

Hi ALL
I converted my 407 to negative earth last year to accommodate a new starter motor (thanks to Greg and Dave for helping me source that) and i simply disconnected the positive earth rev counter at that stage. I thought i'd now get around to converting the Smiths RVI 2810/10 rev counter to negative earth so i can get it back in use and have found several sites on the internet which talk about simple polarity conversion of Smiths rev counters from the early 60s,(although not specific to this actual model). However,although the pictures of the "workings" appear almost identical to the innards of mine, they all talk about attaching the green earth wire to the resistor currently attached to the large spade live connector. i don't have a green wire ! Does anyone know what colour wire currently earths the unit please (or is it a solder circuit connection as i can't see any obvious wire for earthing) ?
Any input most gratefully received.