Most (all?) of the proof marks, etc. for Husqvarna shotguns, shown in standard book "Husqvarna Hagelvapen 1877-1977" are for Models 100 series, 310, etc. guns, not for the underlever hammer doubles.
I have never seen one of the Husqvarna underlever hammer doubles with proof marks on them. I have long understood that these guns were proofed for something essentially the modern CIP standards. In mine, including one from 1900, one from 1920s, one from about 1938, I have always loaded to under 10.000 psi (for "heavy" hunting loads), normally to under 9.000 psi, even down to 6.000 psi. NONE of these gun have shown any loosening with 100s-1000s of rounds of under 9.000 psi loads. All of these guns lock up tightly.
It is easy to load 12s to under 9.000 psi, even well under. Look through loading manuals from powder makers. I nearly always use Green Dot or IMR SR7625.
Unless someone has lengthened the chambers, they are 65 mm. I load cutdown Remington RST hulls and put an overshot card wad over shot to cover hole left from starcrimping shortened hulls (Hartin crimp). With 16s I load Remington black promo hulls, which are actually 67 mm, though marked 70. What Fiocchi 1,0 oz 16 gauge hulls I have used were also 67 mm. Use of 67 mm hulls in 65 mm hulls is allowed per CIP regs.
Niklas
