Re: 1973 Evinrude impeller change need help..
Pizazz,
Are those rectangular black plastic intake screens on the sides of the lower unit? If so, take the 2 screws out and the screens off and see if there's a rod with a nut on it inside. If so, use 2 end wrenches to disconnect the 2 nuts, then slide the top nut up the rod, take off the black plastic cone-shaped "keeper" and slide off the nut. Now your lower unit will come down.
I can't believe you didn't take a pic of the starboard side of the motor!

Anyway, the above is all I can suggest. It looks like you may have a newer than '73 model motor. Can you take a pic of the whole motor on the starboard side and show us?
Edit: Just looking at your photo, that lower unit appears to be much newer than a '73! Your lower unit doesn't even look like the split lower unit like on the '73. It looks like it is the one piece style with the bearing carrier. Are there 2 bolts just in front of the prop--top and bottom? If so, that's the bolts holding the bearing carrier on and definitely identifies your motor as newer than '73, possibly early 80's even.
I'm almost positive you can take it off like the instructions above. Just take out the bolts, take the intake screens off and disconnect the nuts, then get that darned plastic keeper off!
What I can't figure out is the model number.....
Anyway, that is a good looking motor and looks in nice shape. I'd like to see a pic of the whole motor!
later,
Jason