if at all?

"..., if at all, ..."

This modifies "rarely" and emphasizes that the rare event may not happen at all.

Thanks a lot Asherman!

I can "paraphrase" it this lengthy way:

Shifts, specifications, or differences identified in a target text, when compared retrospectively to its sourse text, can only rarely be characterized as translation errors, if at all its source text can be charaterized as translation errors

thak you J-B