Actually, there is nothing unusual about the pencel marking. Unfortunately, the stamp is worthless because of the actions of some postal clerks.
To this day, they still do this procedure on occasion and it is annoying to those of us who like to see stamps 'properly' cancelled, since all post office facilities have the equipment to do so.
At least in the days of the "Sower" stamps, they didn't have felt markers, I think

Some collectors study this particular set for plate varieties etc because different plates were used over the years and some were repaired as well.
My 2 cents worth anyway :)
Chimo
Bujutsu