There was a similar 710 model in the early 1980s. When I handled Apollo, their Dia-Compes always felt mushier than others. Apparently, they skimped on the brake cables, which were prone to stretching. Switching cables made an improvement. Having said that, the calipers do look anemic and new cables may or may not help.

Some Dia-Compe have date codes on the back of the arms.