Well it isn't rare, they were on Dodge smallblock pickups through 1968. While looking this up I came across another bellhousing from a car, not the same casting as mine but you can see it here: http://www.ebay.com/itm/57-DODGE-CORONET...38c&vxp=mtr

Look at the lower attached picture for the flathead 6 mounting pattern. It shares the top bolt hole with the A/LA bellhousing.

Here is an excerpt from Sweptline.com:
"all truck small block bell housings are the
same castiron bell mounted to the crossmember
from at least 62 to 68 (used behind all truck
small blocks, slant six and the old flat head
six- across the line from the flat fender Powerwagons
D and W 100 200 pickups and Townwagons/panels). This truck bell was used in front
of all the manual truck transmissions offered
in those years (3speeds and 4 speeds
as well as some 5speeds).
I assume this bell is stock since it has a
np420 mounted to it (a 420 will not mount
to any car bells in those years). And from
what I can tell from my 66 parts book- and background- nothing
changes for the truck bell in those years."

The 340 pictured has the same bellhousing as was in my '64 and it has the wraparound BH which has the flathead 6 mounting holes in the front flange, ahead of the flywheel.