It does appear from this last photo that there is some sort of securing screw at the top left which is not portrayed in your photo.
Suspicious that Mr Boeing has re-designed the Captains panel securing system but was left with the recalcitrant panels which no longer had to accommodate a screw.
They are still using overhead electrical panels on the 737NG that 'accommodate' the analogue gauges that were installed on the 737-100 and will more than likely be used on the new improved 737MAX!
Waste not want not!