Serial numbers are usually worthless, unless you know the brand first. Even then, many serial numbers have limited value.

Steering tubes are often marked, with a brand of tubing and a date code. Won't tell you the maker of the bike, but it is a start. Seat post size is also a clue.

First picture appears to show some deflection on the top tube, but I am not sure. In person, with a straight edge is the best bet.