Larger for a new caliber is definitely a no-no. Not just because Gemtech asked for clarification either...
If he damaged one and then destroyed it and made a new one to replace it he'd be fine.
As he said, it had wipes and he now wishes to machine new baffles.
To the issue regarding .451 and .458 > It probably had a .470 inch bore or larger to begin with.
Letter? You mean "Form1" and $200 personal check
Sorry, but we've already answered "why". If you wish to build suppressor parts (parts are actually classified as suppressors) you would need to submit a Form1. You aren't building anything when you add a caliber to an SBR. I'm not even sure you can do that after the fact- the reason why people get them engraved "multi" and list them as such on a form1. You CAN later write a letter to add a new length, something that didn't need to be done a long time ago but does now.
I would look into it if I were you. I'm pretty sure VUPD and I are correct but you may get a "good" letter back.