For those started to work before 1992, you are still entitled to

Like my mother who retired 10 years ago, her employer contributed less than 180 months from 1992. But the Chinese SS regulations treated those people as they have made contribution if they worked for state-run or ji-ti companies and you could find the proof (of course you don't need the proof if you retired in China). SS will pay you based on the years of your service jas tui-xiu-gong-zi. In the past, the employer paid tui xiu gong zi to its retired employees but now it is paid by SS bureau.

If you don't have hukou in China, I really I don't know this because I am not familar with the regulations in this area. Very sorry for this. I guess that many oversea Chinese who gave up their hukou may have the same question. 

If you go back to China for a visit, you can try to lvisit ocal shebao ju and ask them to enter your ID (sheng fen zheng hao ma) to search the database. China invested a lot money in this area and each province/city has a very good information system. The company I worked for earned some money from it.