My first reaction is that the '51' might actually not be part of this CDS, but, instead be another marking from the cover that happened to cross the stamp.
The observations made about it possibly being a State abbreviation, or PAID type of marking are certainly very possible, but the overall spacing and orientation doesn't seem correct for either -- at least at first glance. Maybe there is some optical illusion at work here or maybe the month slug is rotated somehow.
I am not aware of any CDS style 1851 year dates; or 1852.
SPQR is correct. There are a few straightlines, like Sonora, CA with that date.
Want to see a more complete "1851" year date?
Keiji Taira and I got some enjoyment out of this item, when he found it around 1991 or so.