This is the first version of the line variation (aka the full line); the second version has the line showing only between ROGER and METZGER, it does not continue into the Giants banner; the third version has the full black line, like the first, but it is faint, a thinner density. It is worth noting that the black line always appears in the exact same space, unlike most stray print line misprintings.
See also Dave Roberts' card in the 1979 Topps set.
PS: the 1991 Upper Deck Milt Thompson is the very definition of a fish eye, print flaw. An aesthetically interesting one due to its placement. It just made its way into the annual guides so long ago that it seems to gets a pass.
Owner and creator of JunkWaxGems, online source for unlisted error, variation and oddball info from the junk wax era.