You supposed correctly

They are to aid the priests memory.
The center altar card contains many of the prayers said at the middle of the altar. These are
-the Gloria in Excelsis
-The Credo
- the prayers before the gospel (Munda Cor meum and Jube Domine)
- Offertory (Suscipe Sancte Pater, Offerimus tibi, In spritu humilitatis and Veni Sanctificator, Suscipe sancta Trinitas)
- the Insitituion ( Qui Pridie to in mei memoriam facietis)
- the prayer at the end of the Mass (Placeat tibi)
The altar card on the Gospel side contains the Last Gospel from the prologue to St. John’s Gospel.
The ‘epistle’ altar card contains
- the prayers used at the mixing of the chalice (Deus qui humanae) - the psalm for the washing of the hands (Lavabo inter innocentes)