I had the same problem and had a look at the updtvpkg script. It sets the version of AIX-rpm to equal the level of bos.rte. The release number updates each time you run updtvpkg.
AIX-rpm-7.1.5.36-xx = bos.rte 7.1.5.36 which is in AIX 7.1 TL5 SP12.
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