It appears Maxwell’s has been expanding not contracting. Maybe consolidating because of age and future loss of branding when Steve reduces his commitment. I doubt his/their success is a product of luck. Not a lot of money in repro hardware Im sure. Probably more focused on larger profit items. Reverb appears to be a questionable platform but I don’t think their website was great by any means. Again, perhaps they are in the process of reworking for the future. Lastly, didn’t Maxwell’s have some sort of manufacturing rights or permission to repro Slingerland hardware? Maybe something changed.
I'm unsure on the market for these parts but for whatever reason it appears he's not replenishing his supply. I'm not sure what sort of quantity he was required to order but I'd expect it would be a minimum of 500 pieces, perhaps more, although it could be less, the Chinese companies who make this stuff seem eager to please. I know that when I contacted Maxwell's late last year (back when they were answering the phone) about a set of these they said they were out of stock but would be getting more in within a couple of weeks. They did and I picked up the three I needed (for a different project). It appears that whatever quantity they got has sold out again, perhaps not to be replenished.
I'm a bit mad at myself for taking for granted that these parts would always be available and therefore putting off buying them. Particularly the repro Rogers cymbal arm he did which was a beautiful piece for $75. As mentioned by others it would have been nice to have picked up a couple of the rail mounts to have "in the bank" as it were. Odd to let that go out of production (if it has) since it is kind of a foundational necessity to do those vintage build kits that Steve specializes in.
The tooling for all this stuff would still exist somewhere in the Eastern hemisphere so maybe they will pop up from another source if someone else asks the manufacturer to resurrect production. It's even possible they might be available now, direct from China, if you plumbed the depths of Alibaba enough to find them.