C087: CC C0 00     LDD #$C000   Set D = 49152 ($C000)
C08A: DD 67        STD $67      Store to $2067

The 2-byte variable at $2067 reveals itself once we observe it with the game in motion. Each time a VSYNC occurs, i.e. for each frame, it increments by one. Therefore, it seems reasonable to call this variable frame_count.

It starts at $C000, but once it reaches $DFFF (after 8,191 frames) it resets back to $C000 and starts again.