Here is a very revealing document written by Laurie Kelso (WBF Secretary) and Jacob Duschek (Denmark):
http://www.qldbridge...ableCalls(JD+LK).pdf
The start of the quote below focuses on a 1
♥ opening bid cancelled at RHO's turn, followed by RHO opening the bidding 1
♠ with a 2
♥ overcall being comparable. Note the last paragraph where it says it is "dubious" to consider a one-level overcall to be comparable due to minimum strength difference being too large between a one-level overcall and an opening bid (if the overcall minimum is 6-7 HCP).
"Obviously, the 2
♥ overcall does not have exactly the same meaning as the 1
♥ opening. The opening bid shows about 11‑20 HCP, whereas the overcall shows about 9‑16 HCP.
The difference in the maximum strength of the two bids is rarely relevant in this auction, so let us focus on the minimum. The overcall can be made with reasonable playing strength on a hand which is just short of an opening bid. The difference in strength, both at the top and at the bottom of the range, is small, and we can accept the meaning as “similar”, i.e., it is a comparable call. There is a good chance that South’s mistake will not influence the result.
Should however South’s mistake nevertheless affect the auction or the play, i.e., if the additional information from the illegal opening bid (which is authorized for North) turns out to be useful for North, the Director adjusts the score.
Note that the Director must not apply UI principles; instead, he must assess the likely auction and play had the illegal call never occurred at all. (This issue is worthy of a separate article.)
The problem is somewhat different if South overcalls at the one-level after an opening bid out of turn. Now the overcall might be made on certain hands containing just 6-7 HCP, and the potential difference in strength could be quite large. Deeming this type of overcall comparable would now be quite dubious."