I don't know exactly what mortar is this...
Looks like M19 by its information.
It may help to do some mortar to Gun Disassembly.

http://pt.scribd.com/doc/15702170/60mm- ... nts-Part-1

http://pt.scribd.com/doc/15702711/60mm- ... nts-Part-2

http://www.cgsc.edu/carl/docrepository/FM23_85_1967.pdf
(M19 Field Manual: there's some tube disassembly on page 8 that can be very useful)