When I got the Shovel, she had 4265's in there.
So this spring picked up another pair, gapped, put in there, ran quite better since front plug was fouled a bit (probably from running with my loose front header pipe when the exhaust bolt napped...just had her wired on end of last season).

Anyways, just ran across a spark plug cross reference chart...and found that the 4265 is recommended for '75 to '81, and the 2585 is for '82 to '84 (mine is an '83)

Can't find what's different about the 2585....anyone know?

Looking at a chart, such as below, seems some brands recommend the same plug for all years, other brands have different plugs for the '82-'84.
http://www.harley-performance.com/ha...park-plug.html

 

Probably the extended tip, which was used in the later years for Shovelheads.
The Extended tip may reach too far into an earlier Shovel...[certainly on an older short reach heads] I don't use Extended reach plugs in anything anymore...But, I suspect the MoCo had their reasons...
Look at them side by side...it will be apparent.

 

Yep , longer electrode off the porcelain , got a set in my extras stash.