The head most likely went bad and was replaced with an automotive equivalent. I have sen many of them but never on a forklift. 15 year cat tech. The head will work but the jet valve was always the weak link in an automotive setup. I have bought and sold many D50 doge trucks that smoked or used oil that was the direct cause of the jet valve. Never had an ill effect bypassing them. The additional jet valves to improve emissions (MCA-Jet system). On the intake stroke, air rushing out of jet valve opening scavenges gases from around the spark plug and produces a strong swirl in combustion chamber promoting high combustion and minimizing exhaust emissions. Not so important for a low rpm forklift engine.