Not much is known of the original vest. It might possibly have been brought to Spain by Clint Eastwood who originally sourced his wardrobe from local shops in America and from pieces of wardrobe he'd used originally on the series Rawhide.
It's been said that the original vest once had sleeves.
The right hand side of the vest is deliberately higher than the left side so as to not impede the character's holster/pistol access. This would seem to be a customization done by an expert quickdraw gunfighter.
The vest is made up of a patchwork of skins and irregular seams lines can be seem throughout its entire surface. It is thin, very roughly and crudely constructed, irregular, uneven, and it is heavily aged.
Most replica vests have a suede-like exterior which this author believes to be inaccurate. From close examinations of the Blu-ray copies of the films this author believes the exterior of the vest to be the bare smooth leather side of the sheepskin.
Some materials spotted on the web that are reminiscent of the vest's construction or styling.