I will try to do some further research, but I have confirmed that Heuer did, in fact, make a Modena chronograph and that the one shown below is a genuine Modena.

I am wondering whether Heuer began marketing it under the Modena name, then switched to Monza because of better name recognition. We have also had postings to the effect that Heuer used the "Monza" name either (1) to celebrate Lauda's victory there in 1975, to clinch the championship for Ferrari, or (2) to celebrate Gauthin's victory there in 1971, in the fastest and closest Forumula One finish in history.

Whatever the reason for the name change, it appears that the Modena itself is identical to the Monza. The sample shown above has had the luminous paint markers removed form the dial, likely because some fell off or were badly discolored.

Jeff