American Express Membership Rewards - Pointless experiment: would NWA lounges accept any Platinum?

As a FlyerTalker, I take pride in my ability to waste time in ways that amuse me, but serve no practical purpose. With that mindset, yesterday as I travelled from MKE-MSP and MSP-SFO on Northwest, I wanted to see if the NWA lounge agents would accept my Delta Platinum Amex, or my Costco Platinum Amex. The results:

Milwaukee: agent almost let me in with the Delta card. Just as I was about to head into the lounge she said, "oh, one second, I need to check that card." She pulled out a laminated sheet that had images of the many (30+) Amex cards that were ineligible for admission to the NWA lounge. She spotted the Delta card, and told me my card wouldn't work. "So sorry," I responded, "here's my Business Platinum". It's a pretty nice little lounge in Milwaukee.

Minneapolis: agent immediately recognized that Costco Platinum Amex was a no-go. I expected that, as this was a high-traffic lounge in NWA's main hub. I did the same "so sorry" routine, gave the right card and enjoyed the very nice NWA lounge at MSP.

Why do I do these things? Just curious, I guess. Amex makes this so confusing with so many Platinum cards, I wondered if third parties such as NWA could really keep track of them.

My friend Ernie used to have the Optima Platinum card, and probably 90% of the time he would get through. It used to look virtually identical to the real Platinum Card.

Then they switched the design on the true Plat. around a year or two ago, and he started getting denied all the time. He bit the bullet and now has an actual Plat, and pays the $450AF. I guess it was good for him while it lasted.

...He bit the bullet and now has an actual Plat, and pays the $450AF. I guess it was good for him while it lasted.

Let's just say I know someone who for three years did something similar with the Amex codes for online reservations at the Ritz-Carlton. Amex finally figured it out, and this person ponied up the $395 to get the appropriate Platinum.

Both of the business platinum cards look similar - if it wasn't for the Amex figure head in the left top corner, I think most people wouldn't be able to tell them apart.