I've recently bought a vintage guitar "Hohner Professional L75". The serial in the back is c103731. What information can this serial say about my guitar? Is it made in Korea or Japan? also which year was it made?
The Hohner L 75 is a Les Paul copy, made between 1985 and 1993. From their historical models document at https://www.hohner.de/fileadmin/documents/service/guitars/Gitarren_Hohner-HistoricalModels.pdf;
The 'Philippines Mahogany' neck is not real Mahogany; it's most likely Meranti/Luan. I think there is a misprint on the price column, and the first price, 445DM (1990) should be 445DM (1985). At the relevant exchange rates, it was $145 (1985) - $293 (1990) new.
As previously mentioned, you'd need to talk to Hohner to get any more accurate dating.
According to the article "How to Date a Hohner Acoustic Guitar", their guitars cannot be dated or otherwise identified by their serial numbers, and one must contact the company directly. This is borne out by the company's website, which has a discussion page for this purpose. A quick perusal of the page suggests that they're pretty reliable about responding, but occasionally don't have the records to provide an answer.