And I managed to identify "mysterious" (for me) short digital transmissions broadcast shortly before or after the analog transmission of the E11/S11a group.
According to Pryiom, this is their digital variant marked with the symbols F03x and P03x. As many as 12 variants of FSK or PSK modulation have been described, where x denotes the letter of the modulation variable.
My curiosity has been satisfied, but I will not report these transmissions.
A sample recording of P03i 1800 UTC 6480 USB S9 U.Twente below:
https://voca.ro/1e93aAhwbMw6