Little is known about Alpha playtest cards.
I have some unsorted pictures of Beta playtest cards, but at the moment I don't recall seeing a beta Counterspell.
Counterspell definitely exists in grey Gamma.
Counterspell definitely exists in orange Gamma. (people refer to these cards by different names as explained above)
Not sure whether Counterpell exists in Legends. Mana Drain might have evolved from Power Sink instead of Counterspell, or it could be a mix of both. Legends has 3 different generations of playtest cards. There's two different little white ones, and then a bigger card sized white ones with the "Legends" name on it.
I expect that Counterspell probably exists in every generation of Ice Age playtesting, but I don't recall seeing it yet. There's grey ones which should probably be labeled is Gamma, for similar reasons to why the early orange ones are called Gamma. The early Ice Age team has been more difficult to contact for info. There's little pale blue ones. Then later there's the bigger card sized white ones with the "Ice Age" name.
I expect that Menagerie will probably also have counterspell for the same reasons. Counterspell is an important part of blue's identity, and these early sets were all based on Gamma, along with knowing that Counterspell was printed in the final version we received. I don't recall seeing it yet though. There's some little textured white ones in between orange Gamma and little pink blank back Menagerie, and we're still trying to figure out whether those were Gamma like the orange cards, or the actual start of the Menagerie project. Then there's little pink blank back Menagerie and little pink Menagerie backed cards. There's the bigger card sized white ones with the "Menagerie" name on them, and a few handwritten big ones too. Then there's some computer printed ones that look more like cards, which is probably around the time the name changed to Mirage. This set was playtested forever, from pre-Alpha until it was finally released in 1996.
Counterspell exists in orange Power Lunch. It probably exists in dark blue Power Lunch, but I haven't seen it yet.
I think Spectral Chaos is fully known. It has 4 generations, little grey, little blue, big white, and stickers. I don't recall seeing a counterspell there, but I don't have everything in front of me at the moment. It has counter cards with different card names.
For the sticker playtest cards, if you search this forum for Portal Playtest cards, there's several to look at. Hill Giant is one that comes to mind. It received a stickered playtest version in Portal even though there were no changes to the card. I think they even kept the flavor text.
This was despite it having been printed in every previous core set, and the sticker is applied to a Hill Giant card.
Basic lands usually aren't playtested though, except for pre-Alpha, Portal because of the missing rules text, and eventually Wastes the colorless basic land.
I only recall seeing one Tempest playtest card. R&D says it's a playtest card, but it's not like most playtest cards. It's something special made in house for presentation to people outside the company. They went out of their way to make it look more like an actual card. It's more like a prototype than a playtest card, but R&D still calls it a playtest card. (It's a big red creature, not a Counterspell.)
Mark Rosewater told me they started a more strict policy of not releasing playtest cards to the public around 2008.
I know I've seen Urza's black playtest cards.