I noticed a mounting hole near my rear spring hanger/ bumperette mounting area. I have no idea what is supposed to be mounted in it. Any ideas? I'm talking about the hole pretty much dead center in the first picture. There is one on the other side too.

It looks like the location where the welded bracket part of the rear valance bolts to. There's one on either side of the rear license plate area. Did you try to match up a valance?

This is right by the leaf spring and the bolt that holds whatever would go vertical into this hole. It does not hold anything with the valence I have, mine has the 2 brackets that have 2 horizontal holes on the rear facing part of this piece of sheet metal.

Tailpipe or muffler hangar hole?

it's pretty far back for that, but my car is a California car, I think they required turndowns which may have mounted farther back... than the standard dual exhaust.

My '72 Cuda had a muffler heat shield that ran front to back on both sides (dual exhaust) in that area. I think that there was a bolt that went into that clip and held the rear portion of the shield up.

On '70 Challengers it's tucked up by the trunk floor and the frame rail.
The rear of the shield is held on by the exhaust hanger. Some type of mount for the California exhaust, seems to be the best possibility so far...but it seems too close to the leafspring.

Only California Hemi and Sixpack cars require turndowns with dual exhaust

My 73 Challenger was just a 318 single exhaust car. It used the same muffler hanger as a dual exhaust car at the back.(plate on the inside of the trunk that held the heat shield and muffler hanger)

1970 used different brackets to attach the rear valance. Do you have the correct style valance on the car or are you using the 71-74 style with the large center mounting bracket?

Just because there is a mounting hole there, doesn't mean anything should be attached there. It might have been used in the factory for something. Maybe it was used to mount that piece to a fixture when the body was being assembled.

My 73 still has mounting holes like that in the rear x-member where the 70 style valance brackets were installed. The factory just never updated the x-member and did away with those threaded holes.

I have the correct (I believe) for a '70 Challenger brackets. they use 2 holes on the back of this part that go horizontal into it. This hole is on the bottom of that panel and the screw goes vertical, just inside of where the bumperette mounts. That big bolt you see is the bolt for the bumper bracket if that helps with referencing where it is.

These threaded holes were used for 2 more L shaped brackets that only came on a few cars. They quit using them sometime in the production year. Not seen on most survivor cars unless they are early build dates. However, I have a 70' 318 car built in Hamtramck with a SPD of 209 that has them. No rhyme or reason why. It's a mystery as to why a few got them or not.
See the thread below for more info. LINKY

Mystery solved, thanks! Mine is a pretty early October '69 built car so this makes sense. Now to try yo track some of those brackets down...

No those L brackets do not go in those holes. Not sure what there for.

How so? Did you read the thread that I linked?

I wouldn't worry about it if I was you. Those holes are empty in 90% of '70 Challengers.

I am not worried about it, but I like the hunt for rare/hard to find stuff though I see now that there are 2 for each side for 4 total. 1 more thing to keep an eye out for at swaps/ebay.

They must have removed the holes for them (or you filled them in) by '71 I don't see them in those pictures. They are just outside of each side of where the leafspring meets the panel.