SOT is Special Occupational Taxpayer. When you pay this tax, it allows you to transfer/build NFA items without paying for tax stamps everytime. $500 per year, but that will save you the $200 tax on NFA items.
Post Sample is a machine gun that was manufactured after May 19, 1986. These post sample NFA items cannot be transferred to individuals.
No law letter means that the FFL is giving up their SOT/FFL and the ATF allows transfers of their post sample NFA items without permission from the buyers CLEO (Chief Law Enforcement Officer) when you are giving up the SOT/ FFL. The reason for this is time. When the FFL no longer exists, the Post Sample NFA item has to go to another FFL immediately. So instead of waiting for the red rape of having the purchaser getting permission from the CLEO, the ATF says transfer it ASAP.
No law letters firearms usually go for a higher prices compared to their law letter required counterparts because some people cannot get letters for post samples (CLEO won't sign off on them) so the no law letter adds value.