Question About Old Coin Flips

This is kind of a silly question, but the modern coin flips I've handled since I started collecting are integrated—I mean the Mylar window is layered inside the cardboard, so they're basically ready to go—just put the coin against the window, fold and staple the flip and you're good.

Among my grandpa's coins, there are some that are in flips that are very different. The plastic windows are much thinner, and it looks like the plastic was separate from the cardboard, and was cut to size by the coin store or buyer and inserted on either side of the coin as it was inserted in the cardboard flip, then everything was stapled together. There are times when the plastic extends out beyond the edge of the cardboard, as if it was cut sloppily.

I'm just curious—was there a time when coin flips came like that, with precut 2X2 cardboard squares with the coin holes pre-punched, and the plastic coming in separate thin sheets that were cut down to size as needed? Anybody have a general idea when the "all in one" flips became the norm? Knowing this could help me narrow down when some of these coins were purchased—not something I absolutely need to know but it adds to the history.

An alternative theory for the flips I'm finding is that maybe they were using old flips where the plastic had been cut out to extract the first coin, and then they were cutting something like Saran Wrap to fit the flip when they added a new coin. I'm less inclined to believe this because it looks like the coins, flips and all, were bought in stores—many have the kind of writing you'd expect—date and coin type (which fits the coin currently in the flip), condition, and sometimes also a price. I'd assume stores would have new flips for their coins, rather than recycling old ones.

Just curious—thought maybe longtime collectors would have info.