From Achim Gratz: 
https://cygwin.com/pipermail/cygwin-apps/2021-November/041670.html

--- origsrc/cygport/lib/src_fetch.cygpart	2021-06-20 09:44:05.617763800 -0600
+++ src/cygport/lib/src_fetch.cygpart	2021-11-14 07:14:23.018207400 -0700
@@ -59,6 +59,8 @@
 #  their basenames to PATCH_URI.  This provides an easy way to carry
 #  Cygwin-specific patches forward between releases.
 #  NOTE
+#  ${PVR}.{cygwin,src}.patch files are special to cygport:
+#  they are automatically applied if present.
 #  See SRC_URI for details about URI handling in cygport.
 #****