Sorry for the late reply.
"If 21 means the additional noise at maximum power,"
I think this is correct, rather than my first guess about the slope. I think the easiest way to confirm is to look at the stock thrusts---all of the subs increased their PSLs by 10 at maximum thrust (5 for the Kilos) so if their c values are 10 and 5, then that's exactly right. Which would mean the unit is the same as the PSL unit as you said.
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