That part is the seal for the HP plug, if it comes from part #5 as you say.
In that case, the thinner oring should work okay IF the inner diameter matches AND it is as hard a duro as the original. Squeeze the oring and see if it is as stiff as the original. If so, you should be okay.
But from your picture, the ID's do NOT match.
If you want to try anyway, to check after your reassembly, just leave the reg pressurized on a tank overnight and then immerse it in a sink of water the next morning. If no bubbles from the HP pay #5, you might be alright.
But I'd look for a new local oring, duro 90, using the chart in that nearby post, or your original and comparing.