I would imagine that it will be a diff-lock seal as the pressurised transmission was only fitted for a very short period, and it was around the time of the 1550.
If I remember right there is a way to know on the very early Unimog's (we're talking 2010/401 here) but there isn't a way on the later mogs, Don't quote me on the 2010/401 bit though

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