C41 RVC is not a specific Corps marking I recognise, and such would normally by found on the heel plate.
The only Castlehead I can find is on the outskirts of Paisley. The 6th Corps of the Renfrewshire Rifle Volunteer Corps was formed at Paisley on 23 November 1859, so although a somewhat tenous link it is the best starting point for research I can come up with!
Rifle competitions and Volunteer matters were widely covered in local press and a search of local papers for Paisley may be worth considering if accessible.
There is an internet discussion list for Scottish Military which you can find at:
http://groups.yahoo.com/group/scottish_military/ Subscribing to the list and seeking further help there may be worthwhile trying.
If you find anymore information please do follow up here.
David