[rob love]

The official answer is in the Gun Drill Manual. I have copied the relevant para below:
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394. Dragropes.--These may be used for a direct pull attached to the drag washers, or a double leverage may be obtained by the use of a wheel purchase.
In attaching dragropes to the drag washers, the back of the hook should be downward, for ease of attaching and because the hook cannot then catch in obstacles such as undergrowth or wire.
In making a wheel purchase, the rings at the ends of the dragrope chains are attached to the hooks on either side of the wheel, as low down as possible on the side away from the direction of the required pull. The dragrope is then laid upon the circumference of the tyre, and the pull must be in line with the wheel. In moving the gun uphill or across heavy ground by wheel purchase, it is often best to move one wheel at a time.
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Note that in the Queen's English, purchase means: noun: a hold or position on something for applying power advantageously, or the advantage gained by such application.
verb: haul in (a rope or cable) or haul up (an anchor) by means of a pulley, lever, etc.

As a sidenote, I ordered in a pair of the dragropes from Tim at Empire trading, and they were here within a week. Mint condition, although there is a variation in the size of the hook from the Canadian dragrope.