You're looking at a factory oil-tank for the 1973 model year 911 RSR. This tank is larger than stock by about 4 quarts of oil.
By the way, the 1974 RSR oil-tank is slightly different in shape to fit alongside the changes made to the chassis-tub by the newer style rear bumper of the 74' 911's.
1973 Porsche 911 RSR Factory Oil-Tank - Photo 1
1973 Porsche 911 RSR Factory Oil-Tank - Photo 2
1973 Porsche 911 RSR Factory Oil-Tank - Photo 3
1973 Porsche 911 RSR Factory Oil-Tank - Photo 4
1973 Porsche 911 RSR Factory Oil-Tank - Photo 5
1973 Porsche 911 RSR Factory Oil-Tank - Photo 6
1973 Porsche 911 RSR Factory Oil-Tank - Photo 7
1973 Porsche 911 RSR Factory Oil-Tank - Photo 8
1973 Porsche 911 RSR Factory Oil-Tank - Photo 9
1973 Porsche 911 RSR Factory Oil-Tank - Photo 10
1973 Porsche 911 RSR Factory Oil-Tank - Photo 11
1973 Porsche 911 RSR Factory Oil-Tank - Photo 12
1973 Porsche 911 RSR Factory Oil-Tank - Photo 13
1973 Porsche 911 RSR Factory Oil-Tank - Photo 14
1973 Porsche 911 RSR Factory Oil-Tank - Photo 15
1973 Porsche 911 RSR Oil Tank (Original) - Photo 1
1973 Porsche 911 RSR Oil Tank (Original) - Photo 2
1973 Porsche 911 RSR Oil Tank (Original) - Photo 3