Gawler's first post office opened for business late in 1842 in the business and social centre of the "frontier" town - the Old Spot Hotel.
Mr Henry Calton purchased the hotel during Gawler's "awakening," and he secured his appointment as postmaster on December 31, 1842. Business was carried out at the hotel until about 1848, when the post office was transferred to the other side of Murray Street to a bakery owned by Mr. George Gozzard.
On Mr. Gozzard's death in the early 1860's, his assistant, Mr. H. Dean, took over as acting postmaster, continuing until the post office and telegraph office were combined in a new building. This building now houses the National Trust museum.
In 1863 Mr. W. Rossi was appointed Gawler's first official postmaster, and some years later Gawler's post office building was proudly unveiled. With the completion of the new post office, the telegraph office became the Gawler School of Mines (later Gawler Adult Education Centre).
The Post Office Department may have erected the post office, but Gawler Town Council saw to the installation of a clock in the tower. Council appointed a committee to purchase and supervise the installation of a clock. A special levy of 3d. in the pound was made to provide the 150 pounds to purchase the clock from Wendt's Jewellers of Adelaide. The clock was installed by Mr. Wendt later that year.
At the Post Office, you need 5 numbers:
R=day of installation
S=number that appeared twice
T=difference between 'incorrect' numbers
U=month of installation
1VWX=year of installation
And now some simple maths to find two waypoints:
Y=U-W
Z=R-W
Waypoint 1: Holland S34 35.WYX E138 45.VSZ
Waypoint 2: Arabella S34 35.RYW E138 45.VTU
Now, GZ is at S34 35.ABC E138 45.DEF where:
A=day of birth of Arabella
B=1st digit of Margaret's age
C=day of death of Arabella
D=last digit of year of death of Margaret
E=50-age of Arabella
F=average of digits of Richard's age
Update: Some of the markings on Arabella's gravestone are beginning to erode and it may be hard to read the date of death. Until this cache is re-done with a different waypoint, the information can be found easily enough online once you have her full name. Alternatively, the Gawler History Society has published the burial register for the cemetery on Flickr, which will give you the information you need. Just be careful with those numbers - date of death and date of burial are very rarely the same.