Not sure how much high knowledge I have of AP actually. 😅 just the basics.
I know they they made off catalog pieces like most other brands … but never steel plots that would fit a 15300. They had an explicit rule for not allowing non precious white metals to be paired with straps. But based on what you have shared, they found the time to machine steel plots anyway for the 15300.
Good on you for being able to get a set of steel plots to fit your 15300. 👏
If you have a picture of your 15300 on strap, do share. I’ve always like the ability to change from bracelet to strap.
Here are a couple of pics of mine. 

🤝