Hi All, I have an old bike I bought on facebook years ago here in Costa Rica, it's my dream to restore so I sandblasted it and I was looking for a serial number on the frame, all I could find was an "H51" engraved on the bottom bracket, I would really appreciate all the help recognizing the frame, I noticed that the top tube has an oval shape, kind of aero maybe? Please help, I would love to know which bike is it or the brand.

We need pics when you can upload them.

Im trying but it keeps telling me i can only post after having 10 posts.........

Your pics are stored in your album. I think this link take us there. You have a few pics so I didn't want to link each one. Someone else (mostly everyone) is better at this than I am but it's a start.

BTW, you need 10 posts before you can directly post pics. You'll be there soon.

https://www.bikeforums.net/g/album/28080860

After I wanted to say "pretty generic alu, can be anything" with those filed welds it is at least something better, not many take the effort even with bigger brands' lower end models. I don't think this info will get you far, it shows similarities to Cannondale CAAD 2 (like several others from the mid/late 90s), but it is not that. Did you drill the holes into the downtubes, or it came with those unknown for me downtube shifters?
Another question, rather for my personal curiosity, was this frame media blasted?

selected pic assist:

It came with those shifters, it was blasted with sand, it was the only placed I found here in Costa Rica, I'm aware that there are better blasting materials such as plastic, but sand was my only option.....

Goodness! sand isn't the issue, it's knowing how to do it, that person did not.
Media matters, but a clean job can be done with sand/glass
Most blasters do a lot of steel and not so much aluminum so they don't retune for the metal.
Firstly, start with an empty pot. whatever your normal blasting pressure is, drop it by 35 to 40%
Always use used sand on aluminum. Second or third generation used is better than new.
Turn the volume up on your sand, 10 to 20% this gives a billowing effect and makes for a nice smooth post blasting surface.
Blasting isn't my living anymore, but I've done a lot of aluminum using sand.

It's sad we don't have that many options over here........ Also, something that might help identifying the frame is that the bottom bracket is Italian threaded......

Here it is a bit too late to feel sorry about, but make sure your next alu bike should be chemically paint removed or by sanding machine. Sand really does not do any good for alu, I mean if it is for daily runarounds and not hardcore prisoner workout then okay, and it is now a really good base for a BADA55 polish... but its not bad in its current form either.
The dropout+derailleur hanger places it to lower shelf even against the somewhat nicer weldings, at least for first sight, but it is a huge community, so others might have a different opinion. If you are lucky it is not from ALiexpress, because then you wouldn't have a chance to find anything similar.