I've recieved a promotional clear black parts container with a recent Shinki order. Does anyone have any info on this? It's the first I hear of it.

Who did you receive it with? When they were introduced (coinciding with the release of Merienda and Kohiru), the containers were a transparent black, switching to solid for future shinki releases.

I didn't know it was their introduction. I did wonder about the different display bases for a while, but forgot about paying attention to when each was introduced XD (The 2 big and 2 small types)
Though I did find out it came with Merienda (which I ordered, along with Espadia) and Koihiru.

Yup, that'll be where the clear ones came from, then.