Just to backup what David said, I have a letter from ATF specifically asking about these LE marked stripped receivers. I presntly don't have a scanner or I would post it, but here is the important part of their reply. You may want to write them and get a letter addressed to you if you are planning on getting one of these as a CYA type of thing:
"A licensed dealer may receive or possess a semiautomatic assault weapon for transfer to qualified law enforcement officers or government entities as provided in Title 27, Code of Federal Regulations, Part 478, section 478.49(c). However, a licensed dealer cannot receive or possess a semiautomatic assault weapon for other purposes, such as stripping the weapon for parts.
A firearm frame or receiver possessed without other components does not meet the definition of a semiautomatic assault weapon, even if the receiver bears the "Restricted Law Enforcement/Government Use." A firearm assembled using such a receiver that does not have the physical characteristics of a semiautomatic assault weapon is not prohibited under section 922(v) ot Title 18, United States Code.
Any person who is engaged in the business of assembling firearms for the purpose of sale and distribution must have a Federal firearms license as a manufacturer of firearms. Assembly of such firearms would incur Federal Excise Tax. Additional information concerning semiautomatic assault weapons and assembly of AR15 style rifles is enclosed.
yadda yadda yadda
Signed: Sterling T. Nixon, Chief, Firearms Technology Branch"
From the way I read and understand this, if you bought a LE/Govt Use marked receiver from a dealer, so long as you had something from the dealer (written receipt?) that stated the receiver was not originally a "semiauto assault rifle" that had been stripped for parts you could legally purchase and put it into a post-ban configuration for personal use. Otherwise, it sounds to me like a manufacturer/FFL would have to assemble it into a post-ban config and pay the excise tax. I'm no legal expert, so I welcome your experience and input.
One more thought, if the receiver ever was a complete LE restricted firearm, wouldn't ATF have a way of researching that by going to the original manufacturer and seeing how the receiver left the factory? At that point, even if you had a CYA letter from the dealer that sold it to you, who would be liable for parting the gun out and would you still be able to keep it?