I can't see from any of the pics on the Sheldon site that closely matches the head sticker on front of the bike. The angles in the pictures have the handlebars covering them.
None of the other models have the same stickers or frame as the one I have. Not sure which year model it came from.

I'm new to all this but isn't a TIG weld very strong. Why does not having a lugged frame make any difference?

It's definately a steel frame, it's also got surface rust on it. It will come off with wire wheel.