That table does not seem to insert a record when Tasks are copied.
But there is a column called PRMETHODEXTID on PRTASK, which is null if a Task is created manually, and has a value = PREXTERNALID if copied from a template. However, the description for this column reads, "Reserved for internal use only. A reference to the prExternalID column in the MRMethod table. This identifies the method from which a task is derived". So, any query making use of this column might not be 100% correct.. I have not tested it out thoroughly either..
Having said that, the following query seems to fulfill the requirement of finding out the template from which a Task was copied, as far as I could quickly test.. You might have to test it further, and may be not rely on it completely..
SELECT ii.name Template_Name,
prt1.prname Template_Task_Name ,
ii1.name CopiedTo_Project
FROM prtask prt
JOIN prtask prt1
ON prt1.prexternalid = prt.prmethodextid
JOIN inv_projects ip
ON ip.prid = prt1.prprojectid
AND ip.is_template = 1
JOIN inv_investments ii
ON ii.id = ip.prid
JOIN inv_investments ii1
ON ii1.id = prt.prprojectid ;