kev74, the different thickness moonclips are meant to work with different brand 9mm ammo and brass. If you search the TK Custom's website there is a chart showing brass brand vs. moonclip size. Unfortunately there are no SAAMI standards on the dimensions of the rim groove width and diameter of each caliber, hence the different models. I think the website also describes the brass brand(s) best used with each different moonclip type in their descriptions.
TK MOONCLIP INFO
Blue steel vs stainless is really about cost and aesthetics. Blue steel are cheaper, but can corrode if you ignore them. SS are easier to clean IMO. You'll see some that have an extra cut between each round; these are meant to be easier to load/unload w/o tools.
I've had about 100 of the blue steel TKC 929 moonclips for 9 yrs. To date I've lost one, mostly because I stepped on it and bent it while moving during a reload. They're really quite durable, but use the tools to load and unload them to avoid.
I believe TK makes the S&W factory moonclips (blue steel) that come with the 929's.