Specifically, it varies by the location the machine was sold.
Generally speaking, the 101-key 1391401 is the safe choice for authenticity for all PS/2 models - but not all submodels, as the submodel often dictates what keyboard it shipped with. There were also numpadless ('tenkeyless') versions, and some more specialized boards for assorted terminal emulation/replacement packages.
The last few digits of IBM keyboard part numbers varies for different regions/language keycap sets and layouts. I believe, for example, 1391406 is the UK layout version.
A couple years after Lexmark took over US manufacturing (they did this in 90-91, off the top of my head, but labels didn't reflect it across all keyboards until a few years later) the part numbers took a format change and some changes...some of these are still 'authentic enough' due to the case being largely unchanged, IBM logo and distinctive click, though.
(as an aside: the 1391401 is what is generally known as the 'Model M', though this is misleading, since the M is not actually a model designation at all...it corresponds more to the switching mechanism within the keyboard...but that's a whole different thing)