Well I say it's a first but BBM has kind of done something similar to this previously. The 2013 Rookie Edition and Fighters sets and the 2014 1st Version set each had two cards of Shohei Ohtani that shared the same number. However neither card was short printed and both cards were considered part of the base set.
The first set to have secret versions was this year's 1st Version set. There were 12 in all - one for each team (of course). It took me a while but I finally got all 12 of them - well I say it took ME a while but actually Ryan tracked down nine of them for me. The other three I got off Ebay - two of them were very reasonably priced while I probably overpaid for the third and final one - Ohtani. Here's all 12 with the normal version - in each case the secret version is on the right:
|2017 BBM 1st Version #177|
|2017 BBM 1st Version #247|
|2017 BBM 1st Version #043|
|2017 BBM 1st Version #094|
|2017 BBM 1st Version #002|
|2017 BBM 1st Version #124|
|2017 BBM 1st Version #201|
|2017 BBM 1st Version #311|
|2017 BBM 1st Version #233|
|2017 BBM 1st Version #058|
|2017 BBM 1st Version #284|
|2017 BBM 1st Version #152|
The first question has a very simple answer. Take a look at the back of the Fighters' checklist card:
The empty circle next to the listing for card #002 (Ohtani) indicates that this card has a secret version. (And just for completeness the "*" indicates that the card has signature parallel versions and the stars indicate that the rookie card has a parallel as well.) I believe that the secret versions can have same plethora of signature parallels as the regular cards.
For the second question...I have no idea. When Epoch took this concept and went nuts with it for their team sets last summer they added one or more stars on the back of each of the variants to clearly indicate if the card was a variant or not. It doesn't look like BBM did anything like that, at least not for the 1st Version set. Here's the backs of Tomoya Mori's cards - I can't see any difference.
The only card where the back is different between the normal and secret versions is Ohtani's - since the secret version shows him batting the back of it has his batting stats:
BBM issued secret versions for 14 cards in their 2nd Version set in August - one player for each team plus Ami Imamura's "Ceremonial First Pitch" card and the Marines team card (the normal version shows the Marines' main mascot Mar-Kun while the secret version shows their new lantern fish one). The just released Fusion set has secret versions for nine cards - five OB players and four active ones (including Ohtani and Hayato Sakamoto).
The secret versions have been kind of fun I'm a bit nervous on how BBM's going to handle this going forward. I worry that they're going to decide that more is better and go complete overboard like Epoch did last summer (and like Topps apparently has). A handful of cards with secret versions is fun and doable - a large number is frustrating and impossible.