Sources of information:
Clark's journal (Louise Patterson)
On National Register of Historic Places:
In Lure of Medford Inventory:
Building is at the corner of Main and Holly, 229 W. Main. Hard to know why the original inventory showed "address unknown"
This website states the building was built in 1936: http://www.waymarking.com/waymarks/WMFXYY_Southern_Oregon_Gas_Company_Bu...
It also has a sketch of how the building would have looked originally.
MMT, 9/11/1936, carried article about the business, didn't mention Clark as the architect.