OK - check out the text string "Hero 616 Doctor" on uTube for various reviews of the large capacity version of the 616 Irridium tipped model of this Parker 61 clone. I didn't know there was any difference, but there is.
I just ordered and received a sleeve of 10 regular 616's...cost me next to zip. NOW I just ordered a 3-pack of the 616 Doctor (all of the above in original blister packs only, thank you). The very slightly different Doctor version was in the neighborhood of $4 ea. delivered. A shade longer to accommodate a longer internal ink bladder.
Note: kinda weird...my father was an MD (Internal Medicine - general practice - did house calls all the time - old school) and his pen was the famous hooded Parker 61. Or it might have been a 51...I'm no expert on vintage Parkers. These thing's official product name being the "Doctor" hits home, so to speak.
Hero 616 Doctor (also called Junbo) photo below: