I am pretty sure that that is an 851. It has the marks that seem to characterize it (
RESEARCH - Early 1960's Pentels).
I just purchased a lot of pencils that should be arriving Thursday. The reason I purchased it is because it as what appears to be an 851 D.
Looking forward to it’s arrival.
I am not fond of Pentel’s early use of heat stamping the information into the pencils. It is sometimes hard to read, especially since they have models 85x and 35x. It got a little better when they started doing the foil stamping, but even that gets worn off after 60+ years.