growing up my Dad had a 50 or 51 8n with a seat support that had a cage with a big spring in the center of the metal cage for a shock absorber. Most 8n seats had a flat spring and i had seen some with the twin rubber shock absorbers. Does anyone know something about the big spring inside a cage seat support (base)? was that a factory option or an aftermarket seat support and around what year? I have seen a few with the big spring inside a cage so they are not uncommon but can't find any info on them. Please help