If I recall correctly that thread, somebody commented that German and other gunmakers upgraded their 6,5 x 52R barrel pressures sometime before World War I to the 25-35W standard. As a a matter of fact my 12x 12x 6,5x52R drilling built before 1923 has been firing Winchester made 25-35s for several decades with no trouble. My CIP Bullet table says that Pmax for 6,5 x 52R is 2450 bar, while Pmax for 25-35W is 3050 bar. Sellier & Bellot sells a 6,5 x 52R at 2200 bar, according to a Frankonia catalog from 2003. So we have both bullets in the market so far. If in doubt I would do some research on manufacture dates for a gun with this calibre and should consider proofing if possible.
Regards,
EJSXS