solahy shutters

Hi, using solahy 3000 those days, one thing i don't get. WIth pan and tilt 0, the shutters make a perfect square in augmentend, parallel to to the pipe. But in real world, i have to give frame assembly -10 angle so that it is really parallel to the pipe. Checked the library, frame assembly home is half the way, so i can't cheat on it.. is this a mechanical trouble with solahy?

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