The UMG's were built specifically for the NYPD. U = 74" side valve. MG = Magneto/Generator. There is a photo of them on page 521 of Palmer's book and 168 of Hatfield's Inside Harley-Davidson.
You see NYPD was 100% pro Indian and 100% against Harley. So they wrote the specifications of an Indian Chief as the requirements for submitting motorcycles for bidding. 74" side valve engine. Magneto/Generator, Right hand shift, Three speed box, Left Hand throttle, ETC. So William Harley took pencil in hand and designed a Harley that meet all their specifications. Not only that but the bid their bike at a very low price. This went on for a couple of years and each year the NYPD was forced to buy Harley's