The original lens were injection moulded, probably into a metal mould that was 100% perfect. Sadly injection moulding is far too expensive for low volume manufacturing and the mould would cost many thousands of dollars to have manufactured. With Injection moulding the plastic is squeezed into mould via one or more very small holes probably only 1 or 2mm in diameter. 

The reproduction lens I sell are made by making a mould from an original lens. The reproduction lens is then made by casting a new lens into the mould. The casting process requires a large opening to pour the plastic into which will leave one side or edge of the lens a bit untidy. This untidy side is always on a side or edge that can not be seen when the lens is installed. Typically the back edge.

Where ever possible I have used New Old Stock lens as the master to make the mould, but some lens are made from used lens that are in very good condition. Either way the mould will replicate any defects in the original lens used to make the mould and possibly introduce a few more defects. When the lens is cast in the mould all of those defects are replicated into the new lens along with possibly more defects. To reduce the number of defects the moulding process used both vacuum and pressure processes at each stage to ensure the best possible outcome.

Very close inspection may reveal very small defects in the reproduction lens but from a few feet away they look as good as the originals and certainly a lot better than faded or cracked lens that have done over 50 years of service.