First off if I have a registered pistol lower does it need to be complete, as in does it need to have the pistol buffer and LPK if I have a sub 16" upper in the house? And if I have one pistol lower or registered sbr lower am I only allowed to have one sub 16" upper in the house?
Once you receive your approved form1 back from the ATF, you can put any length/caliber upper on it you want, temporarly*. No need to mark anything on the lower other than your info.
*What I mean by "temporarly".
You must remember that you filled out the form 1 to build a "specific" SBR configuration, you really should build the gun to match the info on the form 1.(Lets call this your "base SBR config". However, if you plan on making a permanent change to the guns "base" config, you need to send a letter to the NFA detailing the change, they will send you a letter to acknowlegde what you have done and to make a note of it in the registry.
So what your saying is if I have both an sbr upper in 9mm and .223/5.56 and the atf has on file that it is setup in .223/5.56 I could spend a day at the range with the 9mm upper and I would be ok?