This is on my '76 240D, 115 chassis. I'm well aquainted with the injection primer pump (why do my kids always try to use the lower half of the fuel in the tank instead of the upper half??), but what is that OTHER red cap further back on the injection pump?? Looks like an oil filler port, but there's nothing in the owner's manual about putting oil in there. I always thought the injection pump was lubed by the engine oil system. Any clues?

Thanks!