a way to tell 'how old' a fading set of OEM night sights may be on a particular pistol?

This one has "MH3 AL" on side of front sight?

Self-luminous night sight markings usually consist of two or three elements.

“H3” usually appears on all and refers to the element tritium which is a radioactive isotope of hydrogen having three nucleons (one proton and two neutrons) in the nucleus.

All the night sights I’ve seen identify the manufacturer or at least the distributor. TG refers to TRUGLO; M is MEPROLIGHT; SIG seems to be SIG Sauer’s own identification. Trijicon has the full name, plus a “T” symbol with an extra crossbar. There may be others.

The “AL” is a date code, but I can’t find a definitive list of what each code means. They evidently vary among manufacturers, and don’t seem to follow the codes German gun manufacturers use. Not all night sights I’ve seen had anything resembling a date code. The marking on the night sights on a P320 manufactured in 2019 is “SIGH3 AR.” The marking on the sights of a P320 manufactured in 2014 is “TG-H3 S1B”; whether the S1B is a date code or something else, I don’t know.

Thank you sigfreund

As far as translating the codes to years, I suspect most manufacturers are running through the entire alphabet rather than using a single letter to correspond to a single digit like German gun date codes. For example, because A would probably be zero (0), R would be 17. But would a 2019 gun be fitted with 2017 sights? Possibly.
In addition, it’s common for the letter I to be omitted from date codes because of the possible confusion with the number 1. That would push R back to 2016.

I suppose there's a site somewhere that correlated ID# with date of manufacture of the entire pistol? That could be of some help as well.

I would be surprised if gun manufacturers wanted customers to know too much about the night sights on their products. “What‽ You sold me a new gun with three-year-old night sights? I want new wine, er …, sights! New sights.”

I get the impression that many gun owners view their older night sights like the people who believe they’re risking death by ptomaine if they eat a yogurt cup two days after the “Best by” date. Even the newest night sights are going to appear dim if it’s not dark and our eyes haven’t adapted to the dark.

On the other hand, I have a small bag of old sights I’ve removed from guns my agency acquired at least 22 years ago, and weren’t new then. Based on the half life of tritium, they should be about a quarter as bright or less as when they were manufactured. Under nearly full dark conditions and after three or four minutes of allowing my eyes to adjust, the illumination is still visible. It’s dim, to be sure, but I can nevertheless still see it, and would be able to see it better if I had been working the night shift for several hours and hadn’t been exposed to dazzling lights.