Until 1942, whenever a Holy Pope had to be observed at Mass, the celebrant used one of the two Communia Masses for a Martyr Bishop, or one of the two for a Confessor Bishop; or that for a Doctor of the Church, as might be appropriate. The probability is that most of these texts had themselves evolved in the early centuries of the Roman Rite to be used for Bishops of Rome.
In 1942, Pope Pius XII, or his liturgists, were not so preoccuied with the War that they could find no time to rectify this situation. A new Commune, the Mass Si diligis, was composed to be used on the festivals of Sovereign Pontiffs. The same Mass was used for non-martyrs as for martyrs, except for the addition in the prayers of the words "(and Martyr)". It was, apparently, more significant that a man was a Pope than that he was a martyr. More interestingly still, there could be an implication that a Pope was sacramentally something more than a Bishop: which would contradict the ancient and ecumenical verity that there are but three orders of Sacramental Ministry in Christ's Church: Deacons; Presbyters; and Bishops. As Mascall wisely pointed out "whereas the episcopate is a sacramental function of the Church, which is imparted by the sacramental act of consecration, the Papacy is a juridical and administrative one, which is imparted by the administrative act of election". This unhappy mistake, that the Pope is something other than simply a Bishop (in this case, of the Petrine See of Rome), has raised its silly head during this last year in the idea that, somehow or other, something of the Papacy continued to be present in Benedict XVI after his abdication.
But most interesting of all, in the 1942 Mass Si diligis it was appointed that the Praefatio Apostolorum should be used.
Happily, the doctrinal impropriety of this last detail was understood even within the Pontificate of Pius XII, and the Praefatio Communis was soon substituted. But I find it a significant indication of the spirit of the times that such an indecent provision could have been made at all.