For what its worth - the link below is to Leeroy Wisner's site and contains some very useful information about the Miroku O/Us.

http://www.leeroysramblings.com/Gun%20Articles/Miroku_MS_tool.html

This link shows gunsmith B.K. Webster of England making a new spring for a Miroku O/U

http://www.bkwebstergunsmith.com/making-a-miroku-v-main-spring.html