Part # A-40054 - Rear floor Cross Sill Assembly was the official Ford part number/name
Same for all Coupes and Roadster from 1928-31 with the exception of the early 1928 versions which had the three threaded inserts on the left end (as shown in your picture above) for the tail light bracket.
Bratton's calls them the "Rear Body Cross Member" as H.L. indicated above - look in the "subrails" section of their catalogue.
Also sold on eBay: http://www.ebay.com/itm/260915925910?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649
I have a decent original if you're interested PM me.