Ok, idle speculation.
Let's assume a small mech making outfit has a niche market. (Think Coalition Armory, Inc.) They want to expand, but find an 'out of production' design is similar to what they want to market, and would save them a bundle on R&D. How difficult is it, in universe to get the rights to produce an extinct mech or a variant? Think of someone wanting to make MAD-4A Marauder II's