OT Question for RF

There was a fellow who had scads of Hardy Boys (in tan-colored hard
covers) out at the flea market today. My guess is that they were
vintage 1946-1955. When I got around to reading the H.B. in the late
1960s, the covers were illustrated in color.

I've never followed these things. Is there any thing in that vintage
of H.B. that one should look for? The seller wanted $2.50 each,
probably could have bought the lot for less than that per copy.

oly