If you have a new design, then you must contact the ATF Tech Branch at Needy Road and make sure you have the credentials before you submit the design. In most jurisdictions a civilian can legally build a rifle (that includes semi-autos assuming no laws are violated i.e. SBR regs) however if your design is determined to be a newly manufactured "machine gun" then you may be in trouble. If you have an SOT then it provides some wiggle room but I do not think it is necessary, just a safe way to proceed.
On the other hand if you merely seek the letter because you have built your own receiver (re-weld) and you live in an area where you wish to have a piece of paper to give over zealous law enforcement should they show up with questions then your best bet may be to contact TNW and see if they are willing to give you a copy, while they have a newly manufactured receiver, likely the same alterations to avoid it being a "machine gun" as the re-welds were employed, except obviously TNW would simply not mill-out areas that rewelds had to fill in, etc., not sure if that helps.
“… corporations have been enthroned, and an era of corruption in high places will follow, … until all wealth is aggregated in a few hands and the Republic is destroyed.”
- Abraham Lincoln (Republican), Nov. 21, 1864