The 4473 lists receivers as "other." They are neither pistols nor rifles.
If you read the link you provided you will find:
Originally Posted by NICS Fact Sheet
Data stored in the NICS is documented federal data and access to that information is restricted to agencies authorized by the FBI. Extensive measures are taken to ensure the security and integrity of the system information and agency use. The NICS is not to be used to establish a federal firearm registry; information about an inquiry resulting in an allowed transfer is destroyed in accordance with NICS regulations. Current destruction of NICS records became effective when a final rule was published by the Department of Justice in The Federal Register, outlining the following changes. Per Title 28, Code of Federal Regulations, Part 25.9(b)(1), (2), and (3), the NICS Section must destroy all identifying information on allowed transactions prior to the start of the next NICS operational day.
Bold added by me.