Jim King asked:
"There was a post on this group on Feb 20 re: an RRP 36L kit. Which came first, Eastern Car Works or RRP?"
Rail Road Progress.
"Did 1 company buy out the other? I can’t imagine there was sufficient market demand for 2 companies to tool the road-specific N&W H2/2a/3 about the same time."
The two kits were not related. Rail Road Progress kits were essentially a styrene version of the classic Ambroid "box of sticks". You got a small box containing pre-cut sides, slope sheets, hopper components, side posts, and detail parts less trucks, couplers, and decals. There was no injection-molded tooling involved outside of the detail parts. There's a photo of an unassembled Rail Road Progress kit on page 92 of Teichmoeller's PRR Open Hopper book.
The true value of the Rail Road Progress line was that the owner was one of the first to point out that there was tremendous variation in the hopper fleet, categorize them into similar types of cars, and offered craftsman kits to attempt to replicate this. He did a self-published book, "Common Railroad Hopper Cars of the Northeast" to go with his product line.
The ECW model was an injection-molded kit.
"I’ve read that ECW simply “faded into the shadows” and, considering the crude parts by today’s standards, seriously doubt if they’d ever surface again."
The ECW was an odd mix of former E&B Valley tooling and original tooling for prototypes such as their depressed center flatcar and N&W H-2A. Scuttlebutt was the owner was a long-haul truck driver who did the hobby business on the side.
"Does anyone have a “Type 36H” to part with? This is the [e]lusive N&W H-10. Was it ever produced?"
Yes. Jim Six built the Rail Road Progress N&W Class H-10 kits and documented it in the August and September 1989 issues of Model RailroaDING.