what is the purpose of the woodblock in the middle of the bellows if you look close in the image you see it https://www.flickr.com/photos/26984546@N07/53135524678/in/album-72177720310678024/

It's a divider of the bellows.
In the old days they used that and sometimes two dividers.
Maybe they couldn't made the longer bellows ?
I guess that the guy on the photo ordered a new accordion with a divider because he play very traditional.
Bandoneons still have the dividers nowadays , if I'm right.