I'm not familiar with the depreciation rules from that long ago, but *IF* 50 years was allowable, then you should be able select code "O" for the asset type, and then scroll WAY down to the bottom of the Asset Entry Worksheet and fill in the Recovery Period.  It is possible you will need to override.

If 50 years was not allowable and the depreciation period was supposed to be 40 years or less, than as Skylane said, it is over and done with.  But as I said before, I'm not sure what the rules were back then, so it is possible that 50 years was allowable.

Out of curiosity, did they also use a "Salvage Value"?