Ever notice “A7” in our world means 5.25 x 7.25 inches, not 74 x 105 mm? On my W&D 102 I set side guides to 5.375 in and shift the gummer 1/16 to keep the glue line centered on 28 lb wove for this size — does anyone know the history behind that naming mismatch?
It’s the old US “Announcement” A-series, not ISO A, so A7 was made to house a 5×7 card with clearance — that’s why the 5.25 × 7.25 you’re seeing. On my W&D 102, keeping glue centered on 28 lb wove is steadier if I bias the gummer an extra 3/64" toward the flap when humidity jumps, since cross‑grain swell walks the seam; are you running grain long? Handy size chart: Envelope - Wikipedia.
I nudge gummer to 3/32 on humid 28 lb wove — cleaner centers. Source: Envelope - Wikipedia — you seeing flap curl?