Brian, on the back top edge of the mower deck there should be a serial number plate, and the serial number should be in the form of "MXXX-1234567"l have no Blade rotation or Drive motion on this mower. The motor starts and runs good.
Brian, you have an HRB215SXA, and it's a K3 model.l found the number , here it is #MZBA-6311530.
Do not know enough about the mechanical design to understand what source drives the blade.
That sounds like there may be an issue with the Roto-Stop (blade clutch mechanism). If it has any drag or other problems, it could be putting too much load on the engine (when you try and engage the blade) causing it to stall.The engine is still stalling when blade is engaged though.