I rather doubt that the extractor spring from a Mk. IV .38 would fit in a Mk. V .455 because the two guns are very different in size (the Mk. V is much larger). A better solution would probably be to get one of the Wolff general purpose spring packs and trim the appropriate diameter spring to the right length (it's a simple coil spring if it's like my Mk. IV).
http://www.gunsprings.com/index.cfm?...&cID=4&mID=131