Elementary school Mystery series for advanced readers. Late 1960s/early 1970s.

[Dawn]

We had a reading specialist in elementary school that recommended a mystery series for advanced readers to me when I was in maybe the 3rd or 4th grade. I recall the book covers were hardcover and an orange or brown color. I have searched extensively online and cannot come up with this mystery series. I believe there were young boys that solved the mysteries, and no, it was not the Hardy Boys. Does anyone recall a series that would have been around in the late 60s or early 70s that would match the limited information I am able to recall?

[Kat]

Maybe The Three Investigator?
The Secret of Terror Castle is the first book. I believe they came in the same hardcover format as the older Hardy Boys and Nancy Drew books, although I could be wrong. There are a lot of different versions of the covers, so don't go by the modern versions on the series page.

[Mrs.Chanale]

maybe Brains Benton?
https://www.goodreads.com/series/1090...

[Nicole]

There were girls in this gang too, but just in case...Enid Blyton's The Secret Seven series?

[Len]

I am not sure if he ever wrote a series but one author that comes to mind from that period and who was writing for that age group was G.R. Crosher. Books such as Island Adventure, Treasure in the Ruins, Onward Paperbacks: A Bomb in the Submarine Set 1.