Production Run does not mean a single 'line' but instead how many are produced total at one go before paused or retooled. Consider WWII with Shermans . . . ten different factories (or lines) building the tanks all to the same specs and all with interchangeable parts. But production would halt for a bit to incorporate a re-design or upgrade which is why we get the different versions of the Sherman as the war went on- particularly after they had unit testing (cause grunts break stuff in ways designers never imagine) and combat testing/experience. So (just pulling numbers out the arse) the USAA orders Shermans, faucet is turned on!, and starts production . . . . and stops production of that 4A version after 600 tanks from those 10 factories b/c someone introduced a improvement in the engine that increased the life span of the oil filters, primarily desert environments b/c of operations in N Africa. Thus a new production run of the 4A1 which means the production run of the 4A is done, no more ever made! The the engine maker does something to the engine and gets another 50-75 HP, new production run of the 4A2! Meanwhile combat testing says the Sherman needs a better gun and the pointy heads finally delivered so that 4A2 production run gets cut short, just 110 tanks and the start of a new production run, the 4B starts!
For a canon example, NFP rebuilt their Warhammer line on Tematagi to make -7Ms and was producing 16 a month after the first month which was considered a impressive set of numbers. Along with quality control, its how NFP beat Ronin to license the -8D model in the FWL. They started production Nov 1 '67 and shut down that spring on the production run of the -7Ms to retool to the newly licensed -8D model after just a few months- thus ends the production run of -7Ms on Tematagi. The production run of -8Ds started in the spring of '69 . . . though the factory was obliterated in March, so who knows if/how many of that run was made. Btw, it means the -7M was built at a rate of 192 for the year from that factory.
We also hear about major & minor factories, with I think the definition being a major factory is 5 or 6 lines- sometimes producing the same thing while other times each producing different mechs. DefHes on Hesperus? Major factory with a lot of lines even in 3025 though still not producing what it was during the Star League.