Confirmation on that back-gear. External on this. It worked exactly like a back-gear on a lathe with two gears on an eccentric shaft that are brought in by moving the lever. There is a "pin" on the pulley large end to "detach" the pulley from the shaft and make the lay gear available.

And like your "internal" gear Rockford, my current WF&J Barnes DP has a "slider" to hold the stationary part of a planetary type gear reduction in place. (This figures as Rockford and Barnes were like two fingers on the same hand.)

I would assume the HP ratings are based on the presence of a back gear. It's a little unusual to NOT see a back-gear. Mostly on the smaller/lighter DPs.

Joe