The original bolts (ha ha) were set screws that had a long spiral groove for cleaning purposes, they where changed but not sure of when to standard bolts as my 1987 new to me had set screws, however I would go for the bolts as they are a higher tensile strength in this case.
Never fit zinc plated high tensile fixings..... there is no such thing, although this sounds like a contradiction, high tensile bolts are high tensile because they are made from higher grade steels, if you zinc plate a higher grade steel you will get hydrogen inclusion other wise known as hydrogen embrittlement ! google it, it is very scary stuff, as fixings can fail literally like a carrot and with very little force required to do so.
Edited by MRA, 05 March 2016 - 11:44 AM.