This the JPL ephemeris of Comet C/1956 R1 (Arend-Roland) for the night of 1957-05-19/20
1957-May-19 21:00 A 07 03 54.81 +63 27 41.3
1957-May-19 21:10 A 07 03 57.35 +63 27 39.2
1957-May-19 21:20 A 07 03 59.89 +63 27 37.1
1957-May-19 21:30 07 04 02.43 +63 27 35.0
1957-May-19 21:40 07 04 04.97 +63 27 32.9
1957-May-19 21:50 07 04 07.50 +63 27 30.8
1957-May-19 22:00 07 04 10.05 +63 27 28.7
1957-May-19 22:10 07 04 12.59 +63 27 26.5
1957-May-19 22:20 07 04 15.13 +63 27 24.4
1957-May-19 22:30 07 04 17.67 +63 27 22.3
1957-May-19 22:40 07 04 20.21 +63 27 20.1
1957-May-19 22:50 07 04 22.76 +63 27 18.0
1957-May-19 23:00 07 04 25.30 +63 27 15.9
The crude position I gave earlier suggests that mid-exposure was close to 21:35 UT.
Lars: do you have precise postions for the other plates? If so, please post them and I’ll happily do the detective work. If not. it will take me a little time.