Burnsville Mn ?

Well, the good news is the ground is unfrozen. I was a Black Dog Park in Burnsville a few days ago, clad and tabs was about it.

South of the MN river was mostly seasonal farms and pastures until the around the 60's and even then didn't really start to get built up until the 70's-80's because there really wasn't a reliable way across the river until the 80's - the river flooded nearly every spring and left one way to get across (Mendota bridge.)

Better to go across the river and maybe try Bloomington parks, they'd be 50's era. If you're sticking with Burnsville - where Burnsville High School is today, that has been a school (of one kind or another) since before 1900, it shows up on the 1896 map as a school. Also where Neill park is today, the 1896 map shows a school house, the history of Burnsville says that was the first school house in Burnsville... which was called something else back then.

As to rules, there are no rules against metal detecting that I'm aware of, just do a really good job of filling back in your holes and no one will care. I'm new to the hobby and just getting started but mostly people ignore you or come talk to you about what you're finding, how the MD works, how deep it finds stuff, etc.