I found some more evidence that Group 1 (150/350) subjects
might
have been printed with EPDG before any 350 backs were printed and the Old mills were only printed during the 350 series printing.
There are several L.Tannehills that have this red mark in the upper
left corner.
Tannehill SC350-25 - Copy.jpg
I haven't found or heard of any that are on a 150 back
but they are found on
PD350
Tannehill Red Mark PD350.jpg
SC350/25
Tannehill SC350-25.jpg
SC350/30
Tannehill Red Mark SC350-30.jpg
and I recently came across this Old Mill with the red mark
Tannehill Red Mark Old Mill.jpg
1 out of the 4 Old mills I found have it but 0 of the 12 EPDG's had it.
I think this could be important information to the timeline of the
back printing of print group 1 as it has been a common theory
that the EPDG and Old Mills were printed around the same time
during the 350 series printing.