See discussion on SkyTools forum in YahooGroups.com  Four POSS images showing that Abell 17 was indeed a plate defect, which George Abell used the POSS-Ii red plates for his survey for his list.  The other three does not contain this "nebula".  I've enhanced each photo with the same brightness/contrat parameters in Photoshop and posted the photos below.  This photos are copyright from the Palomar Sky Survey.