Fernando VI didn't come into power until 1746, so the piece postdates 1745.
To determine the denomination,take the average of the two radii and double it. If 16mm=1/2 R; if 20mm=1R, if 27mm=2R under F VII; or /28mm=2R under F-VI. If 34 mm=8R during F VI's reign or 4R under F VII's reign; if 40 mm=8 R under F VII's reign.
Don......
I still say genuine also...I would get multiple in-person expert opinions. If it's fake, someone did a REALLY good job AND went to an awful lot of trouble to fool someone with a piece that is historically valuable, but not significantly valuable monetarily.