The NC numbers indicate registration with the FAA - N for National and C for Commercial / Standard. All aircraft in the U.S. have this registration number however in last many years it has changed to a simple single letter and series of numbers.
Here are some images of other Parker corporate aircraft showing their NC registration numbers.
The 1947 Parker Beechcraft D18S so named by Kenneth Parker as the "51" Notice the NC 5151 on wing and tail, which I have been told was a considerable effort, and likely the donation of many "51"s to certain government officials to agree to allowing this designation for the Beechcraft.
All of Parker aircraft had the NC number and also a large imprint of the name Parker painted under the wings also so people could look up and see the greatest name in fountain pens.
The last corporate aircraft were a pair of DC3's that carried the NC designation N51 and N61
All images copyright by GSP Geoffrey S. Parker