Forget.
XH557 was the trials a/c used for the first fit of Olympus 301. First one 301, then two (port), then all four.
XJ784 was the next a/c followed by XL391.
XL384-390 were returned from service and modified to accept Olympus 301.
From the production line the remaining Vulcans, XM574-576, 594-595, 597-612, 645-657 were fitted with 301s.
Ciarain.