I took off my transmission pan to change the filter and fluid. When I cleaned it, I noticed what looks like hammer marks on the inside. The outside does not appear to be damaged in a way that it would require this type of hammering.

Thoughts?

Are those new marks, or is this the first time you've removed the pan?

I'm betting at some point in its life that center got pushed up, and it was removed and beat back to close to where is is suppose to be. The person that hammered it back in place wasn't too concerned about appearance.

The first time I have taken it off.

That's what I was thinking, but the outside shows no signs of serious damage, like it hit a boulder or something. They hit it pretty hard, to leave those marks and they are not consistently spaced for dent removal.

I put a straight edge along the mating surfaces and they are straight, so I painted it and plan on reinstalling it.

Just strange to find. There was very little sludge

Post a pic of the other side.

Here you go.

Looks like there was a crease - center left, extending upper right - and maybe a 2nd crease from the upper rt of the big crease and further up.
Also looks like a few wild swings were taken.

I suspect it was one of those "Oh crap, pull it off, beat it back out, and get it back on, we got to go" repairs.

We had one of those events on our dirt track car once. Got tangled up with someone and got the trans pan smashed pretty bad. We got the car put back together, then dropped the pan, set it on the ground, and flattened it out with about 5 or 6 swings with the BFH while it still had ATF in it, through some gasket maker over the gasket and stuck it back on. Never even lost much fluid. The next week we dropped the pan, changed the filter and gasket, and put it back together. At the end of the year the car was sold for parts. Maybe you got our old trans...

And just add:
I haven't seen everything, but I just can't imagine how a transmission failure could cause that damage.
But also, if it seals run it...