I worked as a mechanic for a Fiat Main Dealer in the 1970's. The original i.e. factory fitted shims were full circle like washers, there was usually 1 thick one fitted over a varying no. of thin shims to adjust camber and castor angles on the front suspension/steering. The slotted U shape ones were more than likely fitted in service afterwards to adjust these angles e.g. after changing ball joints or whatever. They are made from ordinary steel washers by cutting out a section using a hacksaw or anglegrinder to leave a U shape. Use 10 mm hole washers if the studs holding on the bottom suspension wishbone are 10 mm and 12mm washers for 12mm studs ( can't remember which size was used, 30 years since I last worked on one!). The reason the U shaped washers were used was to avoid having to remove the wishbone, compress the spring etc. in order to adjust the suspension angles. You loosen the 2 nuts holding the wishbone pivot pin prise it outwards, fit 1 or 2 U shape washers , retighten the nuts and recheck your angles. It's a matter of trial and error.Hope this helps.
Al.
