I did a quick yahoo search and a bunch of info turned up on your find,there,s a bunch of these on ebay too.Suggest you do that and you,ll probably find the info you seek.Late 1890s or early 1900s seems to be the best bet.Hope this is a help to you.
The serial numbers on these covers should be sequential. The Trident meter was first produced in 1892 and Neptune boasted in 1903 that “Nearly 165,000 Tridents” were then in use. By 1913, their advertising boast was that they had made and sold a million meters in less than twenty one years:
So meter number 389968 would likely have been made some time around 1907/8 plus or minus a bit.