I must have driven past this milepost in front of the Capitol Hill branch of the library a thousand times without ever noticing it - until a few days after completing my article on Capitol Highway and emailing the newsletter.  The inscription on the marker reads "P 7."
 
The three mile marker shown on the 1889 survey maps here marked three miles from the beginning of Slavin Road at SW Corbett Ave.  When was this marker placed and what does this marker mark seven miles from?  No one I checked with at the library or the city knew. 
 
From the beginning of Slavin Road it is five miles to this point.  From downtown Portland south on Corbett Ave to Slavin Road to the milepost is about seven miles, but why have one milepost measured from the beginning of Slavin road and one measured from downtown? 
 
While attempting to determine the location of roads and trails shown on an 1852 General Land Office Cadastral Map by overlaying a modern road map onto it, I discovered that the military road that started on the west shore of the Willamette River opposite Milwaukie looked like it passed this point and that the distance from the river to this point was about seven miles.  Is this the seven miles this post is marking?  You can see my overlay here.  The military road is marked in purple.  If you know, or have any other theories, please email us at newsletter@multnomahhistorical.com.