Elzet,
Interesting that Chris and Tilde Stuart will change their info from correct to wrong, or alternatively Google messed up. I will go with the latter.
I'm sitting with the Afrikaans version of that guide in front of me - the 1995 version, second edition, second printing, and there they state correctly that the record length is 13.34cm. And I say
correctly, because I also have the 1990, 13th edition, of Rowland Ward's
Sportsman's Handbook, subtitled (what a mouthful!)
To collecting and preserving trophies & specimens and containing methods of measurement and other valuable information pertaining to trophy hunting & Rowland Ward's "Records of Big Game". In other words, The Book in which the trophy records are kept. I also checked on their website to make sure that it hasn't changed. And the figure is indeed 13.3cm, 5 1/4", taken in 1971 in Mozambique.
BUT the question is,
which Suni are we talking about, because it seems as if one of them has got the scientific names mixed up, or there is a split that is only recognised by some, as often happens.

There is the Suni, and the Livingstone's Suni. The number I gave here is for the one that is given as having its distribution in Southern Africa. The problem is, Stuart, Apps (editor for Smithers) and Mills & Hes gives that as
Neotragus moschatus, while Rowland Ward gives the Latin name as
Neotragus livingstonianus, with
Neotragus moschatus being listed as appearing in East Africa. The East African antelope they give as maximum record horn length of 10.1cm. I've written Rowland Ward and will see what they respond. Since they classify based on where the animal was recorded, and with the places these are recorded at, I will still say that, whatever the scientific name turns out to be, the length will still be 13.3cm, from Mozambique, as the other one at 10.1cm is from Kenya.
Now, being IT, I'd better give an easy one with no funnies, as from Wednesday lunch I'm gone until either Sunday night or Monday. Will come up with something.