Nice find! On those Mercurys I recommend you have some help the first time you change the impeller. It’s not difficult, except for getting the “impeller drive key” properly lined up while you stuff the impeller in its housing. That is part #24 in the diagram below . Note the shift rod doesn’t show, but everything else should be the same.
Dave
http://www.marineengine.com/parts/mercury_marine_parts/static/img/MERMAR/90694/27.gif
