I hope not too many laugh at this question.

The Model 60 3 inch barrel, 357 Mag, with lock.

When looking at the bottom (underside) of the barrel, about 1 inch forward of the ejector rod, there is a hole. What purpose does it serve?

I guess you don't own many Smith's, Look at what is above it to the right. It is to oil and room for travel on the front latch. No one will laugh at you, now we may laugh with you. Later John

Your mean