The extended forward arm is a traditional British thing BUT NOTE the head is well off the comb, possibly to compensate for the POI of SxSs which tends to be lower than a single barrel or O/U
According to ed's voodoo geometry, that should make the gun shoot even lower
"by moving the forend hand forward...
you are reducing the angle of the barrels...
thus, making the gun shot lower..."

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[Linked Image from photos.smugmug.com]

OTOH, posture/mount/hand position is best left to what works. Some pretty good shooters at the 1896 GAH at Live Birds in various ready positions

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[Linked Image from photos.smugmug.com]

"Blue Rock" Money using a hand guard

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Athol Purdey at Monte Carlo

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