neverguided: "Would it be safe to assume that this rifle (marked 2.9 g.) was therefore likely regulated for the 16 g. loading?"
Yes, I agree. The 2.9 gramm load points to the 16g = 247gr bullet. This should be fine with you, as 250 gr .358" bullets are easily available from Speer and Hornady, while 280 grainers would be an expensive, custom item. Don't be too worried about exact bullet diameter, this is not so critical as some experts make you believe. All the 9x57 and 9x57R barrels I know, about a dozen, happily digest .358" 250gr bullets in front of 47gr VV N140.
A friend bought a Haenel 1900 at a gunshow some years ago. The seller warned him: "It is a 8x57I, so don't shoot 8x57IS cartridges!" So I loaded some cartridges with .318" bullets for him. He came back from the range completely diappointed as he could not hit anything. Well, he couls as well have shot 8x57IS loads with similar results, as the rifle was a 9x57. With the load mentioned above it is a fine,accurate shooter.