Which archive holds records for Swiss Guard serving in Naples, Italy or Vatican?

Question

My great-grandmother was born in Naples in 1853 (implied from the age on her British marriage certificate and British census). On her marriage certificate, her father's occupation is listed as "officer of Swiss Veterans Naples".

Their family name is De Sumichrast which I suspect is a Austro-Hungarian name (possibly Slovakian?). Whilst the the daughter was born in Italy, her parents may be from abroad.

The Swiss Guard are now only to be found in the sovereign state of Vatican City, but back then they were a fighting force deployed across Southern Italy. So who has the archive records? The remnants of the Swiss Guard at Vatican City, or the Italian Government?

I need as much details as possible to trace my family back through Naples to the original part of Austro-Hungary.

The Swiss guard's website contact section to send an email from their web page is broken for me. I'll probably snail mail them but if Stack Exchange users have wisdom might as well tap it.

Answer 1

So it seems the online contact page on Vatican's website does work and I got a reply from Secretariat of Pontifical Swissguard

The Pontifical Swiss Guard can not help you much because the Neapolitan kings had their own Swiss soldiers, which were completely independent of the Pontifical Swiss Guard in Rome. That's why you will not find anything concerning your request in our archive. I advise you to contact the State Archives in Naples, where information about members of the King's Guards may still be found, unless they were destroyed during the Risorgimento in 1861.