From 1991 to 1994 some of the five cent show a distinctly larger version of the designer Stuart Devlin's initials and also a difference in the shape of the S. The difference is quite visible to the naked eye as can be seen on the 1994 dated pair below.

The mint sets from 1991 to 1994 contained just the small SD five cent. Both small SD and large SD were released into circulation.

A closeup clearly shows the difference.

I believe further work is necessary to estimate the frequency of the large and small SD. Going through the stock at work, I found the small SD most common in all years, but my samples were skewed as they contain a large proportion of ex-mint set coins. The 2008 edition of the McDonalds catalogue contains a footnote to the effect that the large SD is rare but it is unclear whether that is referring to the 1992 large SD or to all the large SD.

Wayne Roberts has reported that he believes the mintage of the 1994 large SD to be lowest of the four dates.