Richie Cunningham often wore a varsity jacket on Happy Days.  However, I do not think Archie ever wore a varsity jacket in the 1950s, although they certainly existed at that time.  When was the first time Archie wore a varsity jacket.  I am thinking it is not until at least the 1970s, maybe even the 1980s.

They used to be "Varsity Sweaters.

Jackets came later.

Archie was still wearing the sweater with the R on it well into the 1980s.  I did find what looks like it may be a letter jacket on the cover of Archie at Riverdale High #70 from 1980.  It's got the R on it at least.  There may very well have been earlier appearances, but I just did a cursory glance at the covers of a few titles.

He's had it (jacket or sweater, doesn't matter) sometime in the 2000s, or even the 2010s, hasn't he?  I could have sworn, somewhere. Of course it's not consistent or anything, it's just a whenever-type thing, right?

The Varsity sweater has been a mainstay on covers since Archie #1.  I can't think of many instances of his wearing a Varsity jacket.  Glad Rusty found one example. 

Here's another example from Archie Giant Series #601 - October 1989

Wasn't that sweater/vest thing he wore in the 50s considered a varsity sweater?

  I was only referring to varsity jackets.  I knew about the varsity sweater.