April 8th, 1975, approximately 10 AM. I remember it being a sunny day but cool. I was slouched in a long line, dressed in the usual ’70s dude attire of jeans, black engineer boots, black t-shirt, and denim jacket, wearing a blue cap with the Ford logo on it. I was standing in that line with a bunch of people waiting for a record store in Waterloo, Iowa, to open. Why?

Because Aerosmith’s third album, “Toys in the Attic,” was on sale that day, and we were lined up to buy LPs and 8-track tapes. In fact, I still have the LP I bought that day around someplace, although I have no means to play it. But that, hard as it is to believe, was 50 years ago.

To mark that anniversary, Aerosmith’s producer, Jack Douglas, has told the story behind that album’s top-selling hit, “Walk This Way.”

In the final stretch of making its iconic 1975 album “Toys in the Attic,” Aerosmith stumbled upon inspiration for “Walk This Way” while strolling through the seedy streets of Hell’s Kitchen and Times Square.

“‘Walk This Way’ was the last song that we had to finish,” “Toys in the Attic” producer Jack Douglas exclusively told The Post. “But we could not come up with a lyric or a melody line or anything.”

Taking a break from the studio sessions at the Record Plant in Midtown Manhattan, they went searching for a stroke of creativity in the city.

“Because of the pimps and hookers and drug dealers, there was always a lot of good material on the street,” said Douglas.

But on this particular Sunday afternoon, “it was barren,” he said. “There was nobody on the street.”

This is where Mel Brooks comes in. Yes, really.

“I suggested that we take a break and go see ‘Young Frankenstein,’” recalled Douglas of the 1974 Mel Brooks flick starring Gene Wilder. “And there’s a scene where the hunchback [played by Marty Feldman] says, ‘Walk this way,’ and they all walked this way, which totally broke us up. That was so hilarious.”

And when they got back to the studio, Steven Tyler said, “I got it! He went into the stairwell, and about an hour or two later, he came back with the whole trip.”

Now, that’s a great story. And, in all my years of following rock & roll – the stuff I listened to as a kid is now “classic rock,” and I guess I should be grateful they aren’t calling them “oldies” yet – I’ve never heard that tale before. And I was a big Aerosmith fan, although, in my long-ago concert-going days, they were one band I would have loved to have seen, but never did.