If we may assume that both the 3c and 10c stamps are original to the cover, then in view of the reverse treatment (and the absence of telltale markings on the cover itself) that this cover must have been affixed to something as part of its mailing. It also appears that it was removed from its mount and the notation on the reverse added, as per the observations of Battlestamps above.
However, I'm not sure that the notation on the reverse side is the usual kind of docketing. Here is a closer look:

To me this looks like a name which has been underscored, and below the line the note: "pen and paper"
That seems to me like a reminder note on a scrap piece of paper. Or, more likely, it could have been noting the contents of the parcel on the back of the envelope which had been attached to said parcel prior to mailing. If so, then the postage may have had more to do with weight and nothing to do with registry. Perhaps a slight overpayment for using a handy 10c?
Not sure what to make of it.