I noticed that in addition to the RBT6 and RET6, there's another chip - the STM32F103RFT6.
At first pass, it looks like a RET6 except that it has 96k of RAM, 768KB Flash, and costs ~$1.00 more per chip in batches of 100.
It also seems to have the same pinout as the RET6 and RBT6.
Anyone try playing with these? In principle, I should be able to just swap one in, right?