I think that such lenses were originally sold as either a single 600mm focal length, a two-lens set (600/800), or a three-lens set, with shutters marked appropriate to the set. Likely you have a set that was pieced together from different sets, with a shutter from just the 600/800 combo.
You see the same thing with Nikkor T ED 360/500/720 sets that come up for sale secondhand.
Bob
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