Researching an early Sony product I just found a section on Sony's own website with the history of many of their products. https://www.sony.net/Fun/design/history/1970.html After spending an hour browsing through I thought other people might like to as well. Strangely the TPS-L2 isn't there but its MDR-3 headphones are . I guess it was the headphones that were the innovation as 1970s TC1010 cassette recorder looks remarkably close to an early Walkman. p.s. Nick will be pleased to see one of the the entries for 1979.
Looks like they just upgraded the 40th anniversary portion, somebody must have forgot to do this earlier! https://www.sony.net/united/walkman40th/
I think they updated my link again, check this out a Walkman Digital Map from 1996! I had Microsoft Streets and Trips for my laptop with a GPS dongle but that was around 2004, this was pretty early for this type of equipment.