It is going to take some research to find this address. In rural US, Rural Routes were postal areas, and were determined by the post office. A county would be divide into parts, and each part being a Rural Route. ie RR#1, RR#2, etc. At this time I do not know the boundaries of the Rural Routes in Jay County.
However if your grandfather lived on the property for many years, you maybe able to find his property on the county plat map.
Plat maps from the late 1920's and early 1930's are available at: http://www.ulib.iupui.edu/digitalscholarship/collections/Ind...
Plat Books of Indiana Counties Vol. 3
Page 134 is Jay county
page 146 is Wayne Township with the town of Portland
The townships are arranged alphabetically, so it would be easy to find the township of interest.
Remember the Rural Routes were determined by the Post Office, so one Rural Route (RR) may encompass parts of several townships.
I have been able to get an idea of the location of ancestors by using the latest census, and finding the names of the people that were in the dwellings around my ancestors on the last plat map prior to the census. ie, if the census had jones, brown and doe listed consecutively, and I found jones and doe on the last plat map, it would be safe to assume that brown lived in the immediate area.