I'll try Deaconllb,
But only as far as to add some basic comments on the Contender flat sides from memory or whats left of it (I am not going to go over to storage to try to find my data for any exact information).
The very first Contender frames were plain flat sides with no etched or cast anything on the side plates. I'm talking very, very, very early production in 1967, before the first puma. No idea off hand how many, but not a lot of them.
Another “flat side” was made somewhere in the first 600-700 frames in 1967 that had a defiant eagle. I've heard estimates as low as only 4 to maybe two dozen of them made is all. A version they decided not to put into production.
Next flat side was designed for a chance at being a NRA Centennial Commemorative (early 70’s), appropriately engraved and gold filled. TC didn't get the first one done in time for the cutoff date by the NRA for inclusion, so less than a half dozen were completed.
When TC reintroduced the flat side again via Fox Ridge Outfitters there were several versions. IE blued, color case, etc, inlays, gold/silver filled, several etching choices, plain side for custom etching/inlay, etc. They didn't sell as many as they had hoped at their higher price though. They also offered grips with various animals on the cap.
If memory serves there were also TCA edition flat sides, Ken mentioned the IHMSA version and there may have been more I just don't remember off hand.
Regardless, as Contender frames go they are the lowest run, so naturally the most collectable (plus any lunch hour flat side frames that would be one of a kind). All are probably near priceless to a serious collector except the FRO flat sides, but they would command higher prices as well.
As a side, a hunting buddy bought a FRO flat side and had it full coverage engraved with silver wire inlays that carried over onto the barrel and custom furniture he had made for it. Easily the fanciest TC Contender handgun I ever saw. Gaudy, but actually quite eye appealing as well. And like with me and all my custom Contender barrels/furniture, it was a hunting handgun, not just a show piece. He passed away years ago - no idea where that one ended up but it is as one of a kind as it gets and would be priceless as well.