Assuming we're talking about the same lens, it takes a 72mm filter. I like the Tamron center-pinch type lens caps. They cost less than many others anyway.
And for reference, the outside diameter of the front end of the barrel is 75mm if you're looking for a slip-on cap. If you have the bayonet lens hood, the center-pinch caps can generally be installed/removed without removing the hood, that's why I like them.
As far as I know, the FD rear caps are all the same, after all, that's the lens/camera interface. The official one is a sort of top hat shape with the flange notched to bayonet on to the lens. There's a cylindrical section protruding out of the back end of the lens that I just measured at 48mm, but it isn't very long -- maybe 4 or 5mm -- for a slip-on cap to grab.
That lens is one of the best investments I made in FD series stuff.
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