The postmark on this postcard is a machine-cancel type IV-1 as described in Vatican Notes Volume 44, Number 1, page 17. The outer diameter of each circle is 23mm and the inner circle has a diameter of 15mm. The two circles are 18mm. apart. There is a series of 7 lines separating the two postmark circles. The top line of the postmark moon, or the inner circle of the postmark, gives the hour of the cancellation. The center line shows the day and month (in roman numbers) of cancellation. The bottom line gives the year of cancellation.

The Vatican Notes illustration of the machine-cancel Type IV-1 (shown above) indicates that the measurement of the horizontal distance between the P and E of the word POSTE is 14mm in the left postmark circle and 15mm in the right postmark circle.

Measuring the postcard cancellation of the
left circle shows the P-E distance is 14mm.


The right circle P-E distance measures 15mm.

The image on the post card is one of the Musical Angels by Italian Renaissance master Melozzo da Forli. In 1998, Vatican City issued a set of six stamps featuring the Musical Angels. The post card image was used for the 650 lira stamp:


Musical Angel (Angel Playing Mandolin)
Melozzo da Forli
Scott 1076 (1998)

References:

Rundqvist, Nils, Vatican City Every-Day Cancellations Vatican Notes, Volume 43, Number 6, May 1995, p. 16, (click here)

Rundqvist, Nils, Vatican City Every-Day Cancellations Vatican Notes, Volume 44, Number 1, July 1995, p. 17, (click here)

Hamilton, James, Musical Angels, by Melozzo da Forli Vatican Notes, Volume 68, Number 385, 2020, pp. 8-9, (click here)