Hi love your machine. someday I will stop lurking and really get involved here. Famous last words. I only read when the newletter comes into my box. Just too busy somehow.
Anyway what you have is called by some a class 15 clone. Thats why your bobbins etc from other machines fit. If you can find a manual that the threading layout is the same even some Singers will be the same just look for the tension knob on the front it will tell you everything you need to know. I found the machine below that is another name but the same machine. And below that is a Singer with the same layout.
http://cgi.ebay.com/VINTAGE-CHALLENG...QQcmdZViewItem
http://cgi.ebay.com/Vintage-Singer-S...QQcmdZViewItem
So I hope this makes it a little easier for you.
Good luck with it
Sheila