I agree with Dudley, I have owned many 1 1/2 frame Parkers, all that had HRBPs were #1 plates. Why not measure your butt, #1 plates are around 5" in length, there are some variations, later guns did not use the spur on the plate and I seem to recall that guns made by Remington did not have brothers underneath the dog

I learn something every day, and a lot of times it's that what I learned the day before was wrong