That's really interesting- you always know that components can be swapped and replaced- and even the catalog states they can use different components than what's specified- but to change a specification on the frame- that's interesting.
What's really cool about your bike (that I can see) is the chainstay bridge. I have an 85 620- the chainstay bridge is cast into the bottom bracket shell/lug/casting. That means it's really far forward- it makes mounting a rear fender a bit of a challenge- and that chainstay brace is much more farther forward than it needs to be for it to be a "brace" with even the largest wheel/tire combo it anticipated.
Let's see some more pictures!!!