Often a small hole is drilled for a shackle pin in the backstay crane, In the last three decades or so, many boaters choose to install a rod vang instead. Our boat was built with one, and we immediately decided that if we had known how much better it was we would have converted our prior 26 footer...
(I hope that I have correctly guessed that you are referring to a "topping lift" for the boom.)
My 1971 E29 had a toping lift line that dead headed at the back of the mast head. It was adjusted on the boom. I replaced it with a Garhauer Rigid Boom Vang. That was a Great Up Grade.
Yes, Christian, you have it correct. The topping lift is a double-line around the sheave, but only because of the Dutchman sail-flaking system. The double-line allows vertical/horizontal adjustment of the Dutchman filament lines. The double-line portion of the topping lift has a fixed length, so topping lift adjustment is still done by the in-boom mechanism.