Common error for this date. New master dies where unemployed in this year which seems to be the cause. The master die was changed again in 1972. The full floating roof is common enough not to bring any premium from most collectors. IMO.
The floating roof that you are looking for also has the designers initials missing the FG must also be polished off.
Alurid and Pickin and Grinin make excellent points plus on second viewing the roof "appears" detached but not completely on the east...and there is the issue of the FG..I thought Ebay because of items like yours are listed...you don't want the hassle though! (Buyer disputes)....let the insights above be your guide...