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OAG-L-G3-M68 has a M68x1 thread on telescope side. The back focal distance is 61.5 mm. Note: OAG-L-G3-M68 is manufactured for L size adapter base, so it is compatible with M and L external filter wheels only. While OAG for G2 (with M48x0.75 or M42x0.75 inner thread) for S size adapter base can be technically mounted to S size external filter wheel, the mirror is so close to optical axis, that it partially shields sensors used in G3 cameras and G2-OAG cannot be used.
If the OAG has to be used on camera with internal filter wheel, the OAG is mounted to adapter base like any other adapter. Resulting Back focal distance remains the same.