Hello Tony,
That is a nice looking Police Luger, if marked 1913 on the top, then it'd be just as you said, a 1913 Erfurt, made before they went to the stock lug later in 1913. They were reworked by Weimar police armories in the 1930's, and the Sear safety and magazine safety added. You see more that are "double" dated, the date on the receiver, and then (also) the 1920 Weimar property marking.
In the picture of yours, the top red is the Sear safety, the bottom red is the "removed" and filled in magazine safety, many will be found either removed like this or most of the safety clipped. Both safeties were added by early Nazi, and are one of the few ways to know it is a police gun.
Other ways to tell:
Police (unit) markings to know that it is a police gun.
Police acceptance stamps, such as eagle L on the gun, but no safeties.
Possibly a magazine that is serial marked and has a 1, 2 or 3? But that would make me leery if no other give-aways.
Prices on weimars are usually less than a straight imperial. $700 is a reasonable price, I think $900 is getting higher, not unreasonable for a matching magazine, but I would want it for around $800 at the most, but condition is everything, (as is originality), 90% is okay, and if it turns out to be a bit better, a bit higher in value.
Ed