Can not answer your question, but Brownell's does stock the storage case you need. Maybe a phone call to them will be the charm.
Brownells has the storage case you need in their catalog #53 page 436. Smith&Wesson storage case, Brownell's stock #940-000-555. Factory #226220000. Description: Storage case medium, molded up to 6 inch barrel, all calibers. Price is $10.69
They also have the large that fits up to a 8 3/8" barrel.
Sling Shot