7.62 x 54mm dummy

Does anybody have any suggestions as to the origins of this 7.62 x 54mm cartridge which I can only assume was manufactured as a ‘dummy’?
These cartridges were found amongst a quantity of military 7.62 x 54mm which also contained an opened packet of Kynoch ball rounds labelled ‘7.62mm Russian Ball’. The rounds in this packet were also unheadstamped and, although they were loaded with a different bullet type, the cases look to be very similar.
The Kynoch cartridges have a non-magnetic bullet and an overall length of 77mm whilst the dummies have a magnetic bullet and an overall length of 74.5mm.

For the first without headstamp & primer, it’s a manufacturing Manurhin (France). Test machine loading for Sellier & Bellot. Some are marked S&B and caliber.

JMG, that’s excellent information…and a pleasant surprise.
Thank you.