If your Flying Scotsman is in LNER livery, it is likely to be R855 whereas if it is in BR livery, it is R850. According to Pat Hammond's book, R855 was produced from 1968 until 1977 before being replaced as R845 in 1978 and then R322 in 1980. R850 was in production only between 1968 and 1970.
To determine the approximate year of manufacture of your model, and assuming it has not been modified from its original specification, early examples (1968 to c1974) had all-plastic bogie and pony truck wheels and a light in the firebox to give a glowing effect. In 1974 the firebox glow light was deleted and metal-treaded wheels were fitted to the bogie. In 1980, the firebox casting was replaced by a flat chassis extension casting and finer driving wheels were fitted, with half-depth flanges on the centre wheels which had been flangeless before, plus metal-treaded wheels in the pony truck. These differences are shown on Service Sheets 76, 89, 89A and 89B.