I have been receiving several Delcampe lots from Europe wherein the
stamps are sandwiched between very heavy black card and an acetate
overlay, welded together at the bottom. It looks like an absurdly-heavy
black Hawid, or a cut-up stocksheet of some sort with which I am
unacquainted. My guess is a cut-up stocksheet.

Any pointers as to what this may be?

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Joshua H. McGee, Los Angeles, California, USA
Member: APS, ATA, ISWSC, AFDCS, MBPC, MCC, BPS
President: http://www.penguinstamps.org
Trade?: http://www.mcgees.org/stamp-offers/