The imprint block you are showing is listed in my 2015 ACSC as 211 za, Ash imprint left corner block of 4 with extra hole perf. It is not further listed with any known variety, so I am guessing that this is a one-off, probably a small bit of fiber that touched down during the printing process.
If you were lucky, the same block has two different options: with Variety f, or with variety e and fa. I actually have the one with variety e, but that fa variety is very, very hard to see.

Note: Ignore the typographical error. It says 221e, but this is actually 211e.