Not to be a dick but if the shoulders on the PRL are really 18.75" that is nowhere near a 36 and not even a 38 IMO.
Your not being a dick.
With the P2P at 20" makes the chest 40 - 2 for slim fit(38) or -4(36) for regular fit.
The shoulders are larger than a typical wool suit jacket, but not odd. Shoulder measurements do vary greatly with thicker wools and fabrics like corduroy. I have a 40L Harris Tweed with a shoulder measurements of less than 17" and a Harris Tweed 38L with a shoulder measurement of 18.5"+. I have another 38L older(at least a decade) PLPL slim fit corduroy jacket that should have wide shoulder measurements maybe 19.5".
Edit- Just pulled the RLPL from the garage. It measures just shy of 19.25".
Edit 2- I just realized I haven't worn the RLPL in maybe a year. I will probably sell it this week.
I have several 38R suits that I am considering selling at good prices. (1) A Gianluca Isaia solid navy, beautiful loro piana fabric, lightweight, 3-2.5B, NWT. (2) Cesare Attolini, 38R, 100% super lana, prince of wales gray fabric, 3-2B, light weight, amazing suit, new condition. (3) Kiton, blue diamante, super 100% wool, charcoal with faint narrow stripes, beautiful fabric with great drape, light weight, 3-2B, new condition.
I will be listing these soon, but I figured I would give you guys first crack at them seeing as I know how difficult it is to find 38R in great solid, staple suits. Please PM if interested.