Just noticed that it’s a standard security system aside from the added pull station, nice job! (at least if you put it in) Good choice with the i3 too (are there more elsewhere? There probably should be if there aren’t). As for that one 5816, as long as it works.
That’s a Simplex 2099-9139, which is a rebranded Sigcom SG-42SK. Simplex supposedly rebrands them for outdoor/weatherproof purposes as they don’t make a metal pull station of their own.