If you can find out the thread sizes, bring them to your local hydraulic store. Don't bother with a commcerial supply house, Lowes, etc. If they are straight thread O ring, you can measure the minor inside diameter of the threads and check against a thread chart.
Also if they are straight thread, you can take bolts and thread them in. Obviously only one size will fit properly. The adapter must have a rubber O ring to seal.
A straight thread that only partially enters a port is a very obvious NPT threaded port. The only variances in pipe thread are British (extremely rare, used by Caterpillar) and Standard.
Edit: You can also call Cummins for technical assistance, give them a part number for the part (filter housing, head etc) and they will look up mechanical drawings that call out the port thread sizes. This can also be done on QuickServ with a little time.