Hi Pivi, I have seen pictures of this round and it looks very dubious to me. Yes, it was obviously made from a 81 mm case and the taper of the case mouth seems to match that of the shoulder of the original case. The neck would be too short to support an standard bullet and there seem to be some strange marks in the base of it.
I would like to think that this is an unknown experimental but in my opinion this is a made-up cartridge. If someone ever finds a document to support its existence I would love to see it, honestly.