Well, the thinking used to be that certain items were harder to pick up, up North, compared to what you could pick up, down South.
Brian Rae used to distribute Sue records up here from what I gather and he told me about the above. And that the Edgewood Smith & the Fabulous Tailfeathers was considered at one time to be a really tough one to find / track down.
Suppose Screamin' Jay Hawkins - I Put A Spell On You, could also be considered tough, as I think this one too was possibly unissued. Along with 338, which wasn't issued and as far as I know, nobody has even been able to put an artist or title on it for a release that never transpired.
Other toughies that did get released would include;
Tony Washington
Little (Roy) Milton
Raymond Parker
The Baron.