Under LR numbering system at that time (well pre-IMO) 73xxxxx numbers were issued to newbuildings that were ordered in 1973 and to other vessels that first appeared in Lloyd's Register in that year.
All explained here:
http://info.lr.org/l/12702/2016-07-08/32s57r/12702/146368/45_IMO_nos.pdf

Thanks for pointing that out, Dirk. I've known for a while she wasn't one, I'd just forgotten I'd made that note in the photo's caption, since it has been 7 years since this was posted!

@ Kyle Stubbs; this may not be a World War II US Army self-propelled barge (BSP); rather the vessel was built in 1946 apparently for a civilian customer: http://www.shipspotting.com/gallery/photo.php?lid=3174000
But how do we explain an IMO of a much later date?