The 21C looks a lot like this one:
https://www.nitrorcx.com/sh21nigasenb.html
And this one:
https://www.rcplanet.com/engines/engines-car-truck/redcat-racing-sh-21-engine-redbs801-002/
Also has a plastic backplate.
Can't say I'd spend that much on one though...
The plastic back plate reminds me of the XTM 24.7 engines I used to run back in the day. Was the only engine I have ever seen/had that had a plastic back plate.