The ornamental design for a rear mounted tremolobar, as shown and described.
FIG. 1 is a front right perspective view of a rear mounted tremolo bar;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a reduced scale top plan view thereof placed within the environmental structure of a guitar backside. The broken lines showing a guitar backside are for the purpose of illustrating the environmental structure and form no part of the claimed design.