The width of the drawers of both McDowell & Craig and Steelcase desks is just wide enough to accommodate two rows of 4 x 6" index cards side by side with enough space that one might insert a sizeable, but thin divider between them

I suspect that this is a specific design choice in a world in which card indexes often featured in the office environment of the mid-twenty first century.

Were other manufacturers so inclined to do this? Is there any evidence that this was by design? Did people use it for this? Was there a standard drawer width?

The metal inserts to section off the desk drawer area could have also been used for this sort of purpose and had cut outs to allow for expanding and contracting the interior space.

Keep in mind that some of these tanker desks were also manufactured with specific spaces or areas intended for typewriters or for storing them.