Ok - found it in a post called Passing data as a list vs character vector by jjvalletta
You make a list in R then pass it when you run Stan as in:
fit <- stan(model_code=stan_code, data=df)
Then the stan code doesn’t have to change at all.
After all this, I’m hoping that 2.19.2 is worth the trouble. And thanks for pointing me in the right direction.
Solved.