early pattern hartford units had the outside edges of the steel blades rolled over, later styles have flat blades. between these blades some units had a wood disc only, later this had the addition of the folded edge brass disc which was rivetted to the steel backplate (thus the small centre hole and three rivet holes) also. the later and current manufacture have 'floating' large centre hole brass discs with two wood discs (thick one on the folded return face of brass, thin one on the rolled face). the woods are impregnated with light oil, they need no other coating on re-assembly. note the pressure springs are round ended and change to pointed stars later. the centre bolts change from 9/16" to 1/2" diameter. we only offer one style of silentbloc bush (in three sizes) which all have a steel outer sleeve which should push into the arm ends with a 0.002" interference fit. ensure the the bush end hole is of correct size with parallel inner faces. press these bushes in exerting pressure on the outside sleeve only.