I would sure be interested in seeing pictures of this revolver. I assume it has a 6" barrel, walnut grips, brass front sight and is 7 shot, right? My 2 Trappers both have .22 RIM FIRE stamped on the left side of the barrel and TRAPPER MODEL stamped on the top of the frame. It sounds like yours is a Trapper that was made for a specific retailer and at their request and stamped with Metropolitan Police as a marketing ploy. I doubt ANY police department would use a Trapper.
At any rate, it's an interesting find and I hope to see some pictures as soon as possible.
Jeff in Texas
Walk softly, but carry.