It seems that the real mossie bomber (at least some marks, I do not have XVI cockpit image that I recall) did not have a classic ball like in the VI, but they did have a needle slip/skid indicator. It is #48 in the following diagrams (lower halfs for the bomber cockpits, open the figure url directly to see the original resolution image):
OK I am now legally blind. Both AH moss VI and XVI have this exact instrument modeled in the cockpit.
HOWEVER! my bug report still stands. The slip (upper) needle hardly moves even when in a full slip/skid that puts the ball of the VI all the way to one side. HTC, please recalibrate this instrument. The XVI needs it.