Earth continuity of slot together aluminium enclosure
If you want to make this enclosure continuously conductive (for RF blocking purposes, or for mains safety) then you'll need conductive screws. Another thing that may need to happen is to scrape off the oxide layer (or anodization layer which is a manufactured oxide layer). 

Another way to defeat the oxide layer is with lock washer screws with grabbers and a regular washer between fastener and threads. With countersunk screws (like the ones shown above) it is not desirable to use washers as it raises the height of the screws. 

The continuity can be measured with a digital multi meter (and finding a point with no oxide layer for the probes).