I came across some illustrations of a slinger and some glandes and figured I'd post them incase no one else has.  The book these came from is part of an 'Elementary Classics' series, is titled Jugurthine War, published by The MacMillan Company, and is dated 1921.  Unfortunately, this book was a schoolbook for Latin students and so is written almost entirely in Latin, so I don't know what it says about the slingers.

It is obviously an abridged version of Sallust's Bellum Iugurthinum. Here a  translation of the non-abridged passage into English (I didn't translate it, but found it here: http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Sallust/Bellum_Jugurthinum/2*.... The smiley indicates the beginning of the page you copied):

56 When the Roman general began to realize that he was being exhausted by the strategy of his opponent, who gave him no chance for battle,   he decided to lay siege to a large city called Zama, the citadel of the part of the kingdom in which it was situated. He thought that as a matter of course Jugurtha would come to the aid of his subjects in distress and that a battle would be fought in that place. 2 But Jugurtha, learning from deserters what was on foot, by forced marches outstripped Metellus; he encouraged the townspeople to defend their walls, and gave them the help of a band of deserters, who formed the strongest part of the king's forces because they dared not be treacherous. He promised too that he would come himself in due season with an army. 3 Having made these arrangements, the king withdrew to places as secluded as possible, and presently learned that Marius had been ordered to leave the line of march and go with a few cohorts to forage at Sicca, which was the very first town to revolt from the king after his defeat. 4 Thither Jugurtha hastened by night with the best of his cavalry and engaged the Romans at the gate just as they were coming out. At the same time, in a loud voice have urged the people of Sicca to surround the cohorts in the rear; fortune, he said, gave them the chance for a brilliant exploit. If they took advantage of it, he would be restored to his kingdom and they would live for the future in freedom and without fear. 5 And had not Marius hastened to advance and leave the town, surely the greater part of the townspeople, if not all of them, would have changed their allegiance; such is the fickleness with which the Numidians act. 6 Jugurtha's soldiers were held firm for a time by the king, but when the enemy attacked with greater force they fled in disorder after suffering slight losses.

57  Marius went on to Zama. That town, situated in an open plain and fortified rather by art than by nature, lacked no essential, and was well supplied with arms and men. 2 Therefore Metellus, making his preparations to suit the circumstances and the locality, completely invested the walls with his army, assigning to each of his lieutenants his special point of attack. 3 Then, upon a given signal, a mighty shout arose from all sides at once, but without in the least frightening the Numidians; ready and eager for action they awaited the fray without disorder and the battle began. 4 The Romans acted each according to his own quality: some fought at long range with slings or stones, others advanced and undermined the wall or applied scaling-ladders, striving to get at grips with the foe. 5 The townsmen met their attacks by rolling down stones upon the foremost and hurling at them beams, pikes, burning pitch mixed with sulphur, and firebrands.

Hey the first one's a glans used in the siege of Ascoli Piceno in 91 b.C.!
Its inscription means "Firmum", another town in the South of my Region.
I went at the univeristy in that town! Amazing 
But it has nothing to do with the Jugurthine war 
Greetings,
Mauro.

He looks Trajan-ish to me. Look at the two guys here, if you were to take a composite of them and flip it around for a rear view...

http://slinging.org/wiki2/index.php/Main/TheRomans