gtcsjcode,
There are differences with each light, but from 10 feet you can hardle tell.
-653: it has the numbers "653" stamped on the top and uses a different bulb and bulb holder (these are both on the inside and are impossible to tell)
-656: this is the correct light for 68 Shelby's and the GT/CS. It has nothing printed on the outside chrome and has a different bulb and holder.
-658: I have never owned one but I do know that they where originally mounted upside down compared to the 653 & 656, that is why they have what look like bezeled air slots on the top chrome. It was to let water out if it got inside.
As for all three, I think the lenses have different words and numbers printed on it but as I said above they are very hard to differentiate them unless you get within a foot, especially the 653 & 656. I would not feel bad to have any one of them on my car. I own a NOS set of 653's and 656's.
If you are looking at the ones on ebay, Hans is selling the 656's with yellow lenses (it may have already ended) he is who I purchased my sets from and they are flawless. You can email him and ask if you has any others. He lives in the Netherlands. Here is his email address:
info@italparts.com
Good luck and keep us posted.
Doug