Some ideas. Has a kickstand plate, so likely not a higher end bike - more of a utility type bike? You could try tapping the top tube with something along its length and listen to tone to try and ascertain if the tubing is butted (thicker close to the lugs and thinner in the center).
Measure the exterior seat tube diameter, then also measure the seatpost. Tell us the numbers. My understanding is that it will tell you how thick the tubing is, which can give more information.

We're the bottom brackets standard threading? Also, sometimes the steer tube has markings on it. Did you look for markings when you had the fork out?