The MusicMan amps were Clapton’s rig of choice for a long time. This is a 100 watt screamer. I believe they made these in various configurations and labelled them on the basis of the built in effects. For example, this one is an RP — meaning Reverb and Phaser. There is also an RD version — Reverb and Distortion.
I would doubt any effect other than the reverb is worth using, but MusicMan was a crafty bunch of cats, so I might stand corrected.