Thanks for all the input guys it all makes sense now except for one thing ....
In my suffix G Series 2a 109, the current mph speedo has "1408" written as expected, meaning 1408 turns per mile for 7.50 x 16 size tyres. So presumably the Series 2a km/h speedo (part number 540134) would have a number indicating turns per kilometer, and I calculate that would need to be 1408 x 0.621371 = 875. However, in the listing for 540134 on PA Blanchand's website for this speedo : 
Buy Speedo with Trip SN6147-05A  Part 540134 
I think I can make out the numbers 888. Does that make sense ?
Thanks
John