Found: Kid's Mystery Book - Four/Floor Clue

I'm trying to remember the name of a mystery book I read when I was a kid

I can't remember was it the Famous Five, or the Hardy Boys, or something else, but it was definitely in that genre of book. I feel like their secret den was hidden underneath some sort of mound of tyres in one of their back gardens? That might not be entirely right though

There was one very specific clue in the book which I remember. It was an envelope with two stamps, one stuck on top of the other. The bottom stamp was something to do with money, and the top stamp was something to do with the number four

The group of kids used this clue to figure out the missing money they were searching for was under the floor, because the person who sent the envelope had a lisp, and would have pronounced 'floor' as 'four', expecting the person to whom they sent it to figure out

Any help greatly appreciated, thanks

Alfred Hitchcock and the Three Investigators in the mystery of the talking skull
by Robert Arthur
https://openlibrary.org/search/inside?q=%22Two+stamps%22+four+%22missing+money%2...
Searched for:
"Two stamps" four "missing money"

>1 DH333: I think this is one of the books from The Three Investigator series, but I am not sure which title it was. They had an office in a trailer hidden in junk yard, hidden by the junk. The mail was sent from prison and the sender was trying to give a clue to where a treasure was hidden in the book. But I can't remember which title this was.

>2 bookel: That's exactly the one, thank you

I had a feeling it was a Three Investigators book! I haven't read that one. Went to openlibrary to search text inside books first and am glad it was a quick find. Searching keywords on clues you were sure on made it easy, with phrases most likely to be together in quotes. You can borrow it free there.

>4 DH333: Rereading these as adult made me realise that the settings are really clever. The junkyard makes it plausible that all kinds of stuff appear there and Jupiter will either see every bit of stuff that comes in or one of the workes there will call his attention to it. Also the friendship with the film instructor will sent interesting cases their way. And of course the junkyard van or lorry provides transportation when needed. And the two german workers are handy as strong men in a fight. Etc, etc.

For an added bit of trivia. Robert Arthur's dad was the producer/director of the Alfred Hitchcock Presents tv show. So there was a real connection. I also thought of these books with their base hidden by junk. Not sure which one.

The Three Investigators! I loved that series.

The answer is in 2 above.

>9 bookel: Yes, sorry. I was responding to that :0) Thanks for the reminder of one of my childhood favourites.