It is a boxlock, simple in design. Is it a special kind of boxlock? Is it as solid as alternatives?

If you mean the K Big 5 Classic; yes, solid as a bank vault. Fully as solid and tight as alternatives. Nothing special about it as far as locking goes, only the cocking mechanism is "special". You realize that you only have to cock it once and it stays cocked; some people are confused by that. I have a K 450-400.