I collected these productida brachiopods from the Kibbey Formation in central Montana. This group of rocks and fossils are late Mississippian. Location is slightly north of the little Belt Mountains. On Highway 89 north of Riceville Rd.
This type of produtida brachiopod appears to have a bundle of spines projecting from the pedicle and scattered spines projecting from the shell. The spines are very long and about as thick as angle hair pasta. The spines are more or less straight and are as few inches long. The photo shows both the pedicle and brachial valve. The valves many times are deeply wrinkled and folded.
Cool fossil! I am an amateur like you. I did some searching on the web and found an article about brachiopods with spines. Perhaps the author has the knowledge you are after. Also there are some references about identifying similar brachiopods you mentioned. Good luck hunting let me know what you find out.
Yes, I am familiar with the woostergeologist blog and I have read the post on the permian productid brachiopods collected from Texas. Those he writes about are a little younger. With brachiopods knowing the age of the formation where they were collected is really important. But there are a huge variety of brachiopods forming a confusing hay stack of almost look a likes. Ages and locations narrow the possibilities.