The statistical odds of picking up any Smith-Corona M'03-A3 rifle at random and finding a 6-groove barrel are about 1:30. I'll let you be the judge if that's hard to find.
You can dramatically raise those statistical odds if you selectively pick only low serial numbers with early barrel dates. But randomly finding a low serial number with an early barrel date is factored in to the preceding odds.
Hope this helps.
J.B.