Glad I found this topic... I am somewhat of an expert on ESL products myself.
The older ESL line of detectors were made from the early-mid 1980s to the late 1990s. in both 2 and 4 wire variants, being 425 and 445, respectively. There were also similar detectors sharing the same cover (here's a list of all of them!)
-422 (2 wire)
-446 (Canadian 445)
And yes, there is a date code on the units. It is 4 digits, usually followed by a T. The first two correspond to the year it was produced, and the latter two are the week it was produced.
I used to have a 445CT installed in my house, too, and I replaced it with a 449CT.
Another important point: There has been an issue with these older 445 series detectors "going bad"... most of the time that is not the case. The sensing chamber on these units is designed in such a way that makes it especially prone to false alarms from dust. Frequent cleaning can prevent this, but by the time they fail, the 10 year life would have already expired, and it is time to get a new one anyways.
A lot of info, but hey, I like to help out as much as possible.