ABRO was the Army Base Repair Organisation. http://en.m.wikipedia.org/wiki/Army_Base_Repair_Organisation
"The plate says the engine was rebuilt for ABRO, which means it was done by an outside contractor. There were a number of diverse companies doing this work as well as some ABRO workshops. ".
Army service would also explain the date discrepancy; the earlier date is likely to be when Land Rover assembled it, the later date is when the Army disposed of it, and it first gained its civilian registration.
If you want to know the type, 11J, 12J, etc, then the guys here will need some pictures to show timing chain / belt cover, overview of the cylinder head, and a view showing any diagonal stiffening ribs on the side of the block, or that there aren't any!!