"...listed as non-restricted...installed a different barrel..." Entirely possible. Iver Johnson made some 19" barrel carbines for a while. Just for Canadians too. (The Carbine has been restricted since 1978.) It might be one of those. However, like contact148 says, check it for an extension.
They are more expensive compared to what I paid( so long ago I forget what I did pay. Less than $150 though), but $300 to $400 is about right as I recall. Get yourself into the Exchange forum, here. You might find something there.
If it says Universal on the receiver, there were two models. Early and Late. The Early models are ok. The Late models are junk.
A Late model will have two return springs(requires removing the stock to see), a 'toggle' for the bolt hold open and a really badly stamped op handle(they break). Late models cannot use ANY military parts either. Universal thought they were improving the design and made a horrible mess of it.
"...year of manufacture..." There's no web site that has a list of manufacturing dates by serial number for any carbine. Milsurp or otherwise.