When he was studying in Israel, my father purchased monogrammed tallit bags in Me'ah She'arim for himself and for his father, ז”ל. On both bags, the form used is "Ploni b"r (ben harav) Ploni". neither my grandfather, nor his father, both zichronam livracha, had semicha, as far as I know, although my grandfather was a chazzan. Why would the embroider use the title of rav if that is the case?
Similar to what Shmuel Brin mentioned in the comments, I would guess that the R' stands for Reb. That seems to be somewhat of a Hebrew equivalent to 'Mr.'
When I was in High School and the Gabbai for the school Minyan, the Rosh Yeshiva said I should add the title 'Reb' before the names of non-Rabinic adults (i.e. I called my friends up as Ploni ben Reb Ploni and other adults as Reb Ploni ben Reb Ploni). In your case, I guess the R' was added to display an additional level of respect for their fathers.