They used that 13.5 grain load back in the day. My suspicion is that the cases had greater capacity, and/or the primers were a little bit milder.

I worked up to it one time, fired a few in my model 27. Not ever going to load any more. It was right on the ragged edge, and I shot them on a fairly cool day.

Lots of fun, but I prefer a slightly milder load, thank you.