Your cost basis for the purpose of depreciation is the *LESSER* of what you "originally" paid for it when you purchased it, or it's FMV on the date placed in service. I have no doubt that the lesser amount is what you originally paid for it when you purchased it.
You can't "unbuy" and then "rebuy" something to yourself. Your original acquisition date remains the same. Doesn't matter if it was 50 years ago either.
