Hi,
The Davis Sewing Machine Co. went bankrupt in 1924 but National used the Davis name for some time afterwards. I'd guess the the machine itsself was badged "New Davis". According to the Needlebar this is a model 52F:
http://needlebar.org/cm/thumbnails.php?album=260&page=7
Nice find, the warranty would greatly interest those researching National.
Jon