Originally Posted by T-Mar

I have an April 1979 comparison test stating that the Le Mans employed a 5 speed freewheel, specifically 14-17-20-24-30T. Given the time to get an article to print during the era, the test may have been conducted using 1978 models. Consequently, it would appear that the Le Mans was 10 speed through at least 1978 and there is a good probability that the current freewheel is OEM.

If the OP wants a higher degree of confidence regarding the originality of his freewheel (and the model year of the bicycle), he can start checking component date codes using the information here; Date of Manufacture of Bicycle Components can be used to date a bike: component dating . I suspect he'll find late 1977 to very early 1978 date codes, which would be consistent with a 1978 model.