The 4.1 was made from machined material, which is a 'proper' manufacturing technique. But...as a relatively low volume technique it would have been expensive to make, and only made in small numbers. Not ideal maybe, but it was a means to an end for a company setting up doing a conversion kit to test the water with. They all have to start somewhere!
Now they have a proven platform, they have invested more heavily and the 4.2 gearbox is moulded...so could be made by the thousand...you 'should' be able to get the 4.2 spares for a long time to come!
Welcome to the real world of product development, that is all!