Both shows, (in addition to another police show
that is a favorite of mine, "Kojak") were made
by Universal Studios. They, at least in those
days, must have had character actors under contract,
so they would shuttle them from one show to another
that they were producing. Thus, it is not suprising
to see props go from one show to another, either
(the painting of the fictional detective lady in
"Murder by the Book" appeared in an episode of Rockford as well). Universal made a golden era of TV in the early 1970's, having top actors, directors and
facilities.

I know the reasons but it's fun to see, and I thought i'd let everyone know.

The most notable Columbo-Rockford connection that comes to mind of course, would be Gretchen Corbett (Beth Davenport on Rockford) as Janus's secretary in "Exercise In Fatality."