Ancient Saddles

[demagogos nicator]

How much do we know about using saddles in RTR timeframe?
I was under impression that saddles were quite rare during this period.
From what I have found about this topic the fourhorned saddle which is on the picture above was invented by Celts and adopted by Romans in th 1 century BC. Thil this time most of the cavalrymen of "civilisated factions" were ridding saddleless horses.
In contrary, in game this saddle is useed by almost every cavalry unit except Celtoi, Celtiberian and Lusitanian Heavy Cavalry (only generlas bodyguard cavalry of Celtic/Celtiberian factions do use saddles).
On what was based the redistribution of saddles to the units in game?

[demagogos nicator]

Nobody knows?