Can anyone date the use of the grey plastic stock/grips that were used on the Model 5 (and Model 6) air pistols following the change away from the 1958 - 1960 wooden grip types?

What I would like to know is how long this grey colour of plastic stock/grip was used before the brown plastic types that we are more familiar with here in the UK were introduced.

Lastly - does anyone have a plastic Model 5 or Model 6 that they would like to sell at a fair price ?