Fixed the very same bug in two OpenPGP implementations: SKS and pgpdump. Both didn't handle some subpacket types correctly. This field gives the type of a subpacket, for instance 2 is signature creation time, 3 is signature expiration time, etc. The issue was that bit 7 has a special meaning. It's not part of the number but it's a flat, saying that an implementation should be able to handle this subpacket or treat the entire signature invalid. Patched both and submitted them upstream. Now sks finally shows signature expiration times in its vindex.