Just purchased a 1948 series 2 Colt Woodsman. I haven't shot it yet. Just noticed the metal (takedown)back strap pushes in slightly at the top.
When I pull the slide back to cock the gun the top of the back strap pushes in about 1/16th of an inch against the web of my hand.

Is that normal?

Any ideas what's wrong?

Normal no worries. The hammer strut enters the mainspring housing which is removed for field stripping and has abit of wiggle to it.
Go to rob Rayburn's woodsman page to learn more than you will ever need to know about woods man pistols.
I have a 1950 era challenger similar situation and completely normal.
Spare magazines are a bit hard to find and expensive- if you get lucky finding them expect to pay no less than $45 each and even $75 is not unrealistic.
Very accurate and reliable guns- I was out with mine the other day and was hitting 8" square plates consistently at 65 yards with a one hand bullseye stance

Okay, thanks captain127 .

Glad to hear it's normal.