That throttle was first released in 1968 & was available in various colours over the years.
They were a standard 35 ohm resistance.
I still use them on my old layout. They`re a lot better than the modern throttles.
Variations in performance could just be down to age & ammount of use.
There was a very large batch of throttles sold circa mid 1970s which are potentially dangerous as they have the heat shield missing. If you have blue or yellow throttles from that period look through the top vents to check the shield is there.....if not, chuck it....
(between 1966 & 1972 there was a larger, black race tuned throttle, available in either 15 or 25 ohm)