For parts like mufflers, some electric switches, etc. that are somewhat "generic" I would suggest that Lotus got a batch of them to build cars and for some spares and, in the case of mufflers, would would modify them for the particular application. When those ran out and the design of the muffler itself was likely changed over time, some suppliers probably took it upon themselves to reproduce those parts to maintain a "bolts-on" capability. Shops like SJSportscars and Lotus Supplies (or Europa Engineering) in the UK and r.d. enterprises (and likely Dave Bean Engr.) in the US are examples of this. Commonly, they are provided with uprated materials. I just looked at Lotus Supplies' and SJSportcars' websites and they both provide stainless steel mufflers for S2s and S1s. In fact, curiously, SJSportscars lists the muffler for the S1 as a "Lotus Genuine Part" but the muffler for the S2 is not (to my eye, they look VERY similar so I suspect that designation was left off by accident).
So in some cases the best you're likely to get is a part that might "look like" and probably "fits" or "mounts like" and "works like" the original but might but could be easily identified as not by someone who would know. Sadly, there are fewer and fewer of those who would know because of father time!