If it’s the 55th Elizabeth bodied Monaro then that would be probably a June 1968 assembly so would definitely have a straight inner guard behind the headlight and an offset booster probably without the dent. If it’s the 55th GTS that is a fair bit later so may have the dent for the booster but probably still an offset booster. If it’s the 55th GTS327 that is heaps later.

That is a reasonable time after initial build, remember Elizabeth built less cars than both Pagewood and Dandenong. To get the right answer though on lever versus straight brake booster you really need to find a reasonably original car close to your build number to see what it has. Ben and Warren stated in their book that sometime in September 1968 was when the straight rod type booster began to be installed.