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.
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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.
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.