37 Repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into automobiles and structural parts therefor; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into loading-unloading machines and apparatus and structural parts therefor; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into construction machines and apparatus; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into agricultural machines and implements and structural parts therefor; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into metalworking machines and tools and structural parts therefor; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into mining machines and apparatus and structural parts therefor; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into plastic processing machines and apparatus and structural parts therefor; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into rubber-goods manufacturing machines and apparatus and structural parts therefor; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into air vehicles and structural parts therefor; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into pneumatic or hydraulic machines and instruments and structural parts therefor; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into plowing machines and implements and structural parts therefor, other than hand-held tools; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into brakes, machine elements for land vehicles; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into power transmissions and gearing, machine elements for land vehicles; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into mechanical elements for land vehicles; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into oil pressure controls for non-electric prime movers, not for land vehicles; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into liquid pressure controls for non-electric prime movers, not for land vehicles; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into fluid pressure controls for non-electric prime movers, not for land vehicles; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into hydraulic controls for non-electric prime movers, not for land vehicles; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into valves, machine elements not for land vehicles; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into brakes, machine elements not for land vehicles; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into power transmissions and gearing, machine elements not for land vehicles; repair, maintenance and conversion in the nature of installing hydraulic energy regeneration system into machine elements, not for land vehicles