Ah, understood. Never seen that before. Strange that an album with different label and catalogue number still is considered the same release. I would have expected it to show up as a separate release to pick. Obviously, that was a naive thought 
In that case, I see 3 possibilities:
- Put in the 1st/2nd/… (just a choice I suppose, I guess 1st is OK)
- Put in all (likely to get too long)
- Put in a placeholder (“multiple labels”) and let the person do it by hand
I suppose that it will not happen that frequently, so I think (3) might be best. What would you suggest yourself?
Thank you