Note: The Leyland Parts Catalogue lists BHH1026 for chassis numbers 294251 (Aug 72) to 386600 (Sep 75) and BHH1851 for 386601 on including for the 77 to 80 cars, Googling shows neither except references back to BL documents. Moss Motors list three different air-vent pipes (hoses) bringing fresh-air in to the valve during running to purge the canister of previously absorbed fumes. The 1970-74 drawing lists 367-280 for 73-74 and the 1979-80 drawing lists 367-290. But the 1975-78 drawing lists three including those two with different dates plus another - 367-280 for 1975-78, 367-285 for 1976-77 (those two date ranges overlap so maybe the first one should read 75-76) up to chassis number 415000, and 367-290 for 1977 from chassis number 415000 to 78. All are NLA so there are no pictures showing how they may differ, but the 1970-74 engine bay and exploded parts lists shows a short pipe (item 64) going downwards, whereas the 1975 -78 and 1979-80 engine bay shows a long hose coming forwards but the exploded parts list shows the same short hose as before. However John Twist in this video indicates the hose on a 1977 going to the front of the car through the radiator diaphragm panel, and a pal with a 76 has this long hose (oil can for scale) which has never been fitted in his ownership: (Bill Etter)

Bill has fitted an oil cooler and there is no third hole in the diaphragm panel for the valve hose, so maybe this long hose is supposed to use one of the unused cooler hose holes.