I have a 1959 Cessna 172 with a MDM-7655 prop on an O-300-A engine.
I was given a DM-7653 propeller.
Both props are listed as the cruise props according to McCauley's application chart, and both are approved on the TCDS.
I would like to send one prop out to be re-pitched for climb.

Is there a noticeable difference between the "MDM" series and "DM" series propellers?
Which prop would make a better climb prop?
Or, is one of the two designs better suited for cruise and I should leave that prop alone?

Macaulay is a fantastic company with great product support. I would call and ask them for the information that you are looking for.

The 8 bolt hub is the DM
The 6 bolt hub is the EM - this prop has a different/improved airfoil over the DM

The MDM is the EM airfoil on the 8 bolt hub.

The DM7653 is not equal to the MDM7653, with the MDM being slightly more of a climb prop than the same pitch DM prop.

The MDM is a slightly better prop, but barely noticeable.

When you have an anemic O-300 pulling 2400 rpm on takeoff, a 20 rpm improvement is noticeable.