Yup we have the same frame. I dont have the forks.
My serial number is 77F0405
So since they made over a hundred of these someone must know what they are.
Mine had older then USA Kuwahara decals on it (pre 1979 font) so I thought maybe Japan Kuwahara.
Perhaps these are 77 Sekais?
I know by 79 Sekai was pushing hard in the US. Maybe they started in 77 ?? Look at the similarities to their 79 bike. Both their 79 and ours are looptails, both have box behind BB, both have a front gusset (yeah in 79 they went to a more modern Mongoose like but in 77 may have used a more Redline like single gusset).
http://sekaibmx.wordpress.com/I think both your U's are 0's. Stamping on mine is also not closed but thats just the stamp, looked closely and it was clear they were meant to be 0's.