Meaning of $ within LTspice netlist
It could be that it's just an atypical character that's used simply because it's allowed and it stands out. The line starts with X, which is the designator for subcircuit. Whatever characters follow afterwards have to meet one single criterium: they have to be allowed by the parser (i.e. no errors should appear, the circuit should run).
$ and the M afterwards might suggest that it's an artifice simiar to what LTspice does when it substitutes elements, behind the scene. For example using [Misc]/Epoly with the FREQ keyword (bogus example):
E1 out 0 FREQ {V(in)} (0,0,0) (1,1,1) (2,3,4)
makes the expanded netlist to show up this line:
B§E1 out 0 V=V(in) FREQ (0,0,0) (1,1,1) (2,3,4)
So in this case it might be that some MOSFET (which M designates) might be involved, as in there was a schematic which had a subcircuit mimicking an MOSFET (it had the designator M), but LTspice detected it's not a .model and, when the general subcircuit was created, someone replaced the § with the $. The not labels seem to agree. It could be, it could be not. But the end result is that it's an allowed character and can be safely used; nothing to worry about.