The "MULTI" caliber marking is nothing to fret over. There are people out there who have some notion that you can't change the caliber of an AR-15 if the lower is marked as a particular caliber. Marking it Multi gives those people peace of mind that they can build a .22, .300 BO, or 6.5, if they desire. It also serves to silence those who might flip out if they come across someone with a 5.56 marked lower attached to a different caliber upper (range-Nazi RSO's, etc).
Up until about 10 years ago, BATFE NFA Branch would accept Form 1's for short-barreled AR's with MULTI in the caliber box, since the caliber can be changed so easily. They stopped accepting them around that time, though, saying that what was listed on the Form 1 was what the rifle was to be built into when it was first made and that you couldn't have two different calibers at the same time. Changing calibers after the rifle was made into an SBR was fine, but the Form 1 has to reflect the condition it will be when it is first built into an SBR.