Something is strange about the marks. Regarding German marks, 142/148 & 144/148 would be a plug gauge and right off I'm not familiar with it. Some Express rifles were in 24 bore(0.577 inch). But as far as the stamp being 14.2 mm would tilt toward a Belgian origin. Grundig(.. over u) I believe was a retailer in Dresden who dealt in dura-frame or alloy type lightweight frames. I would like to see a pic of the action and the marks to verify though.

Kind Regards,

Raimey
rse