WHAT IS IT?

I need help with this cover as I don’t have access to the book which may give me more information about this interesting postmark and registration label. The blue Australian 3d KGVI head stamp is postmarked RAILHEAD P.O/ -1 AU 40/ R.M.1. The blue registration label R/50 has a typed Trav P. O/ A I F / 780, and the other significant postal marking is a red boxed PASSED BY CENSOR/ [crown] No. 606, with 2 manuscripts ‘XXX’ and ‘No 24', both of dubious importance. The cover is addressed to a post office box, at the Sydney suburb of Chatswood, N.S.W. The reverse was not seen (Figure 1 ).