I'm almost positive that is the Poldi mark, there's a sheet somewhere on the site with the image, on the tubes of the all but true auction offering. This would go a long way in revealing that the maker stamped the tube type on there and could easily mix and match or stamp whatever the customer requested. I'll bet since it had "Krupp Steel" on the top rib that either in Germany or over here someone realized that it also need a Krupp steel mark on the underside. With Linder in control, I'll be it wouldn't have occurred.

Kind Regards,

Raimey
rse