What's the deal with Microgrove LPs?

Microgroove is a term from the 50s.The first records after the 78 were 10" to fit the shelving which already existed for the 78.The 78 has a wider groove.The new groove was narrower therefore called microgroove.This is the same size groove used today,so play on.On some 50s-60s turntables you had a flipover cartridge with a 78 stylus on one side and a LP (microgroove) stylus on the other side.

7" of 50's are always 45rpm and there's no issue with current sound pickups. I'd be cautious about condition of these not to damage your speakers with loud clicks pops that may occur.

Thanks, but there seems to be some disagreement as to whether they'll play at 33 or 45. Not a real big deal, I'll figure that out easy enough. As long as there are no issues with today's cartridges, but as Czarivey admonishes: I'll be careful.
To Mattmiller: Yes, they are pretty thick.
To Ghosthouse: You are correct, it should be a single "O" not Groovy!!
To Elizabeth: I think you are also correct. They may have been freebies b/c old authie did belong to a record club as some of her LPs state they're from the club.