Distributor cap question

[sprbxr]

I have ordered a distributor cap for my 911 a couple of times. Every time I get one it doesn't have the locating lug on the under side of the cap where ist sits on the distributor. Anyone know what cap to order? One of the problems is I don't know what distrbutor is on my car. It is a 70 911 with a 2.7. I guess It is a hodge podge of various years.

Justin

[twin plugged targa]

If you have a digi cam you may want to post a pick of the dist cap or try and list any markings/numbers to help the guys here.
These guys will pick your car apart and fault diagnose from 1000 paces!
Hope this helps
Ben