It seems like they chose to not have ruled beams on any of the edges. I mean to say that it was a conscious decision more than an oversight.
Here’s a thread about that from late last year that prolly has some more info (sorry, I only found it, I didn’t reread it)…
It seems like this has come up several times, and Dan has suggested just making some if you need some. A reasonable solution, IMO, though I do like the precision offered by a factory-made part.
Here’s an example of making them being suggested…