The original version which came pre-installed when the computer was new was OS X v.10.7 Lion. I do not have the original DVDs - I do not recall ever having them but, if I did, the have been lost in inter-continental moves over the last years. I have tried Network Restore using "Reinstall Mac OS X" from Mac OS X Utilities: that takes me to the screen "Mac OS X Lion" and I click on "continue" (twice to check my computer's eligibility); that takes me to the licence conditions to which I agree; I select my HD, then sign in to download from the App Store with my Apple ID. This tells me that my ID is valid only for the Hong Kong store and I will be switched to it; I select OK. Then I get the "This item is temporarily unavailable. Please try again later."
Of course, it is NOT available later. No matter how much later one tries.
This seems to be because, if I log into the USA Apple site I am told I need a "redemption code" at a cost of USD19.95. That's rough as the system clearly recognises that I am entitled to download the "unavailable" software - though I suppose that a company cannot keep servicing an old product indefinitely. Be that as it may, if I try to buy the redemption code I cannot do so as the USA site only allows for sales to USA residents (which I am not) and there is no similar facility available on the Hong Kong Store site.
Stalemate. And hence the question I asked...