The MOD54 reel allows you to develop up to 6 sheets of 4x5 film (I usually developed 4 at a time) in daylight in a 3-reel Paterson System 4 tank with 1 liter (IIRC) of solution. I bought the Paterson tank and an original MOD54 reel, a multi-piece glued-up design, when the reels were first introduced. The original reel's edges seemed a little sharp so I carefully smoothed any edge that might contact film. I later bought one of the newer one-piece molded reels, an improved and more robust version still available new.
I used this a dozen or so times with the original reel, but it never grew on me - I prefer processing in trays in the dark. I don't recall that I ever even used the newer-style reel. This gear has been taking up shelf space in my darkroom for long enough, it's time it moved along.
B&H sell a kit consisting of the current-production reel and Paterson tank for $110, or the reel only for $65. I would like to get $75 for the complete Paterson tank (tank, lid, funnel, center column, and agitation rod) and the two reels, all in excellent condition. My asking price includes US shipping and PayPal G&S fees.