[Mr70]

As has been mentioned,there have been a few original documented solid lifter Chevelles/Elco's found with the wrong hydra lifter tachometer installed at the assembly line.Your's is not the first,nor I'll bet the last to be discovered.
I've heard of a few 5500 redlines hydra's getting the incorrect 6500 solid upgrade too,but I've yet to see those in person.
A 3 & a 5 do look similar to some respect.
A few L-78 1970 Chevelles have also been found with incorrect 5500 redline tachs still in them.
Rodney C's Chevelle,and Chuck Hansons 1970 L-78 Red/Red convert too,who re-installed the proper 6500 during resto.