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It sounds like someone put a later rear axle in the 51 but it could still be a Dana 44. If it has 19 spline axle shafts then it would be a 1955 or older D44. Lockers can be put in the D44 but those are aftermarket parts. A person would have to find out what unit it has to find parts for the locker set up.
They do look like thrust blocks for a D44 Detroit Locker. They are available from several sources for around $25. A lot of lockers are for the later 30 spline axles.