I believe the BATF changed the classification to "Other Firearm" for stripped lowers and pistol grip shotguns. I also believe that complete lowers may fall into the same category but don't recall for sure.
The biggest change to the purchaser this caused is you now have to be 21 to purchase a "Other Firearm".
I don't know if purchasing multiple lowers at one time is required to be treated as "Multiple Handgun Purchases" since the above change and documented on a separate record.

Good question.