The fact that there is a mention on here of that 601/602 ejection port cover on GunBroker and after a day it is still available sorta tells a story. This is the place to get the word out about stuff like that and generally if something rare and hard to get turns up posted in this forum it doesn't last for sale long. Time will tell what that story is.
I do disagree a little with the statement that the only reason more people don't build 601/602 clones is because of parts availability. Yes more of the serious retro clone builders would put them together if the parts were more readily available but the fact is most people are building M16A1s and XM177s because that's what they carried, or that's what their Dad carried, even now in some cases what their Grandfather carried, or they just think it would be cool to have one. It's what they primarily saw used in the military for the 25+ years. Not near as many people had contact with 601s and 602s. And most of the people building even the M16A1s and XM177s don't even clone them 100% they just want a close representation. Most of them have the wrong upper receiver for their era, and most are on a A2 lower receivers, and some have the wrong barrel due to twist rate, profile, or length. But they still want it to look more or less aesthetically like a M16A1 or XM177 from a few feet away so they need that small rectangular pad ejection port door. It's why so many people show off pictures of their rifle with the door open. It's not because they have a wonderful bolt carrier it's because they have the wrong door and want to distract from it. I on the other hand always picture my rifles with the door shut to show that I have the right one. I know, I know, a lot of us can tell from the inside of the door too but most people can't. People could build 601/602 clones that way too, meaning a representation of one using some parts that give the look but not 100% correct, but they just don't on as large of scale as the others because it's just not what most people want or even know about.
It really just comes down to ones own perception, what I think, or you think, or we think doesn't really matter, it's what the other people out there are thinking that drives the market and the demand for certain items.
I have to admit I kinda like seeing items sell for a lot from time to time because it just makes me feel like the stuff I have is worth even more then I thought. Whether that is true or not is a different story.