Howard, you may email me at
donmaus1@aol.com.
I looked at the Simpson posting and it doesn't say where the unit mark is located. I have seen them on the front grip strap and on the front of the mag well. The mag well location probably indicates an earlier mark.
I will go out on a limb: The mark may be "Op." and not "UP." I have found that most people are not that precise when reporting police marks. I have also seen an instance of an Op. mark that was reported as OP. because the lower-case p. was displaced upward and looked like an upper-case letter. If so, this mark is from the Prussian governmental district of Oppeln.
My real collection is my unit mark database. My Luger collection numbers only 3 but is about to jump to 5!