I kind of question whether they were working at home but instead were probably working in small shops similar to the link below.
I'm not sure how old theses photos are of DK's works but it's probably a good glimpse of how many of the makers operated.
http://www.daiichiko...ojyougazou.html
If you copy or open each image in a new tab you can view them at a larger scale.
Even the old Nihon Seiko building was not that large. Most of these shops were small. These were not factories. even if you visit Cumberland optics in MD that building is not that large. Optics manufacturing depending on the amount of sales is generally pretty small considering we are not talking about sales in the millions of units.