(Update: This post has just been pointed out by nimble-fingered reader Kim. Sadly, it seems Denis never received a reply. Maybe the scientists are still mulling over the 'other details' before responding).
(Update #2: Simon has pointed out this article. The 'scientists' have agreed to reply to Denis. I'd call that a result).

Transcript
Envelope:
TO A TOP SCIENTIST AT
Woomera
ROCKET RANGE
South Australia
URGENT
Diagram:
MY ROCKET SHIP
AUSTRALIAN MARKINGS
- Pilot Navigator etc.
- Radar ANTENNA
- Radio ARIEL
- AIR TORPEADOS (GUIDED MISSILES)
- 4 Rolls Royce JeT ENGINES
- Jet can be fired individully
YOU PUT IN OTHER DETAILS
Message:
PLEASE WRITE ME A LETTER BACK
HERE IS A ROCKET SHIP DESIGNED BY
DENIS COX
26 CHUTE ST.
MORIALLOC
VICTORIA