Need help in identifying this Girard-Perregaux

Short of opening the case back, does anyone know of this GP model and likely year? Its an inherited piece and so beautiful to me. Its 20 mm wide, 38 mm lug to lug, 10.5mm domed crystal height, and the strap is a 10kGF Topcaps60 stamped Speidel expansion band 15mm.

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Anyone? Still looking for any help on this one?