The IBM 5100 computer from 1975 is apparently the first “portable” or personal computer. The 1981 PC from IBM was the 5150 model.
I learned from an improbably site – The Time Travel Tale of John Titor which provides posts from a purported time traveller from 2036 who came back looking for a 5100. (Thanks to Matt K for the link to this.)
The John Titor site may be part of an art (or fiction) project … but then again, he could really have travelled back in time.