How come i can only get 2 port, ported vacuum switch for my 1978 c10 350. Oe part has 3 port brown in color. Lives on thermostat hihousing.

[1978C10]

How come i can only get 2 port, ported vacuum switch for my 1978 c10 350. Oe. part has 3 port brown in color. Lives on thermostat housing

 

[C10MixMaster]

the hard part is figuring out the part numbers on emissions parts. My 77 used a 3 port switch . the numbers i have for it in my notes are Wells E9121 or standard PVS6. it might be a ebay part.