I presume a 4473 is a criminal background check.

The federal government is free to disregard state expungments, but normally they do not. A good example of where they do honor it is for firearms. If the state restores your gun rights, the feds will honor. A good example of where they won't is immigration. Under the Immigration and Nationality Act, a conviction which has been set aside based on a finding of rehabilitation is still a conviction.

If you have a good faith basis to answer "no," I would answer know because the algorithm probably bounces you if you answer "yes." In the GOES portal you have the ability to provide additional information. There I would answer something like: "While I used to be convicted of a the offense of dinosaur rustling, by virtue of my 1969 expungment of the offense I believe the answer is now 'no.'" By disclosing all the operative facts, you cannot be accused of deception, by you will at least force a manual review of your application.

I can tell you that the website for NEXUS says you can join with a pardoned conviction. Pardons can come from administrative agencies, not simply the chief executive officer of a state. That is what Georgia does and that is what Canada does. If you want to appropriately research the issue, I'd start with the Code of Federal Regulations on this program and see if it defines it with greater clarity.