I've been interested in Penzel-Mueller for a while (I got a P-M clarinet years ago, before you could research these things on the internet, but never took learning to play seriously until recently).  I have read through this thread a few times as a guest. It seems generally assumed that Penzel-Mueller stopped in the late 1950s or 1960. I found these interesting P-M price lists on Reverb (I don't collect memorabilia and it's not my item): https://reverb.com/item/35945607-penzel-mueller-1968-and-1970-price-list

The price lists are labeled "68" and "70" and the seller calls them from 1968 and 1970. That might be a leap, but the address on the first page uses a 5-digit zip code. ZIP codes were introduced in 1963 and not everyone used them right away, so it roughly supports the seller's dating, or at least says that Penzel-Mueller still existed in the mid 1960s. They advertised a line of several clarinet models and were importing/representing Borgani saxophones and a variety of other instruments.  An image of the 1970 pricelist is attached.

The lists are also marked "E. Chiassarini Division" - not sure if P-M was a division of Chiassarini or the other way around. I can't find much about Chiassarini other than that an Evaldo Chiassarini imported reeds in the 1920s.