I agree with you. I think that, on the whole, the pre-heresy legions were much much less into the whole religious ceremony and trappings of faith that are part of the 40k world. The pre-heresy legions saw the emperor as the father of the primarchs and as the greatest man and greatest psycher to ever live. But, they didn't see him as a god, except for the Word Bearers.
For a pre-heresy iron warrior or iron hands marine, I would think the only true deity they would believe in would be the omnissiah the machine-god. For that reason, I don't think that they would use parchment or wax seals at all. Religious trappings would be so much nonsense to them. The only true worship to the machine-god would be to perfect the effeciency and power of their technology.