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The trans in your image is a 1950 V8 Oldsmobile, one year only trans, same design as Cadillac 1937-54 with differing tailshaft lengths. Those use a standard three-speed floor shifter for use with transmissions of standard two rotating lever sliding engagement design. The 54-56 you ask of is a selector trans used in Oldsmobile, Buick and Pontiac which uses a different, unique design of shifter, to accommodate the single revolving, and second sliding lever design. Those floor shifters are a lot harder to find. That guy Heathen or heathen1960 on ebay has them for sale fairly often.
The most common example seen today are those formerly made by Ansen. A google search of 'ansen selector shifter' returns many image examples, here is a clear image seen installed on a 1954 Pontiac selector box, which also looks to have beefed up linkage. Hope you find one.