I'll grant you that the prop was modified over time, so it's difficult to know just what the details of it were during its tenure as Vader's ANH dueling saber. But that little front t-track piece survived all the way into production of ROTJ when the saber was cast for the severed hand gag, so it's not a stretch to suppose the unidentified greeblie box was on it during ANH as well.
I'd call the pic inconclusive, if anything. It is hard to tell. Luckily we have the surviving prop, and (even better) the cast ROTJ hand gag prop that allows us to make an educated guess as to what was there.
Looking at the first two pics Sym-Cha posted, it looks like t-track on one side, and a faux MPP release on the other.
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