So, what could it mean? The most plausible explanation is a case of mistaken identity. There is another highly popular design data book used in India and globally: the , formally known as the PSG College of Technology Design Data Book . "PSG" could easily be mis-remembered or mis-typed as the number "31". Alternatively, a searcher might have conflated two different resources. The esteemed Machinery's Handbook has a "31st Edition", and it's conceivable that someone searching for a portable design reference mixed up the name with Bhandari's.
In digital terms, "portable" usually refers to a standalone digital copy or a compressed file format that does not require heavy software installation to open and view on the go. A Note on Copyright and Legal Access machine design data book by vb bhandari pdf 31 portable