In the meantime I could luckily watch the SFM drawing #7031 of 1891.
The ECRA caliber database calls this drawing as only reference for the 7.5x58R.
In fact the drawing shows no 7.5x58R, just a normal 8x58R case. The special feature is the bullet:
It’s of solid steel with a driving band of copper.
The diameters are 7.7 mm at the steel part and 8.34 mm at the driving band. The last fits the standard 8x58R case.
Focussing on the smaller diameter is perhaps the reason why it became mistakenly a new caliber?
And, of course, the dubious 7.5x58R pictured here, has nothing in common with the SFM drawing.
Sorry for the bad news, treshkin.
@Fede: Do you have any further information about the remarkable danish ‘7.5x58R match’? I’ve checked different danish documents and sources and found out … nothing. Could it also be a chimera?