Equipping Units (Addition); SIFRP, Page 173
At the end of the third sentence, add the following:
(Cavalry, Infantry, Mercenaries, and Personal Guards all carry shields).
Equipment Upgrades (Addition); SIFRP, Page 173
At the end of the paragraph, add the following:
Alternatively, you may spend 1 Wealth to equip a unit with shields, thus enabling certain advanced orders. Should you do so, the unit does not gain an increased to Fighting damage, but instead increases its armor rating by 2. Units that have shields (see above), cannot be upgraded in this way.
The consequence of this is that Archers with new-bought Shields have better armour rating than Infantry (who is supposed to have shields by default). That is as good as Infantry upgraded on armour, but without the bulk.
Other comparisons reveal similar flaws.
It seems to me that the units already having shields should have a higher Armor than given in the table, or the units without shields should have their Armor reduced or the shield bonus should be reduced.