Thanks for the additional pics. Those are marks for the Gorham Corporation of Providence, RI. Despite the appearance, I wouldn’t doubt it to be Sterling silver… unless it’s an outright fake of a Gorham piece, which I doubt.
Gorham used ‘pseudo-hallmarks’ in imitation of English hallmarks. The English lion for Sterling is shown facing the wrong way, the anchor is an imitation of the city mark for Birmingham and the Gothic letter ‘G’ (for Gorham) is styled as if it were a date letter. Several American companies used these kinds of pseudo-marks to create an illusion of the prestige with which English silver was held.
The marks are in a style used from 1853 onwards, but Gorham didn’t adopt the Sterling standard until 1868. At the same time, they began a system of year-marking using letters of the alphabet starting with ‘A’, as ordinary serifed ‘Roman’ letters, until 1884. They then switched to using symbols between 1885-1933, and then single digit ‘Arabic’ numerals from 1941 onwards.
I don’t see a year-mark of any kind, so I would guess the tankard was produced between 1934-1940 when year-marking was suspended. When it resumed in 1941, year-marks were only applied to “important” pieces, so the tankard could also be post-1940.
I can see a crude hand-engraved Latin ‘XVI’ below the marks; also a digit ‘8’ and perhaps a letter ‘V’ below the ‘2355 code’. I don’t know what those are, but they’re not in the formats Gorham used for year-marking. If one theorised that ‘XVI’ was an alternative shorthand way of indicating a date of 1916, it’s not something I have ever seen on a Gorham piece. The year-mark for 1916 was an axe, shown upright with the blade facing left.
Here's the set of marks from another Gorham tankard which also has the Latin numerals ‘VII’ crudely hand-engraved. So, I would assume that they were applied in the Gorham workshop for some kind of quality control or traceability purpose. Note that this tankard also has a year-mark (as ‘O’ for 1882) but it’s crisply stamped from a punch and not hand-engraved, as would be usual.
I would hold to a likely date of 1934-1940 (or later) for your tankard, and the ‘2355’ code is a style number. Note that the other tankard I imaged has a higher numbered mark as ‘3647’, but style numbering wasn’t necessarily sequential in the sense that the lower numbers indicate earlier pieces.
Bargain price for that amount of silver!