Yeah it's entirely possible the version that I have that is supposed to be from 1937 was tainted with later versions despite it not containing any of the more well known 1951 changes. That is maybe someone reconstructed it by taking a 1966 copy and undoing the changes, but forgot about the small slimy creature change.

But apparently there were dozens of different versions that actually ended up in print that had different amounts of the changes caused by some printers mixing old plates and new. So it's entirely possible that small slimy appeared in some versions around 1951 but not others and that's what that page is working off of.