The difference between your glow plugs and later ones are mostly in the wiring. The later plugs are fast glow, which means they have a changing resistance as they heat up. With low resistance cold they draw a lot of current heating quickly and changing resistance so the current drops to normal levels to maintain the heat.
The later plugs are also much smaller in diameter and are wired in parrallel instead of series. This means each is a separate circuit and the other plugs will continue heating even if one fails; unlike the original system with stops all together with a open circuit.
There are fast glow kits made for 240Ds of the same era and I have seen people use them on 300Ds. I saw one recently that even came with a glow relay for replacing the whole system. We usually keep one in stock as it is the only way we know to get these plugs and there are a number of cars that have been converted. Ask me on Monday and I can get you a number.
Bosch Master, ASE Master, L1
33 years MB technician