What are these "nonstandard format" lines in Distant Artificial Satellites Observation circulars? Mike G's great answer clued me in on how to find the collection of astrometric observations of spacecraft which is maintained by the Minor Planet Center. 

I've noticed that there are nonstandard format lines embedded within the observations. I can read the Date, RA, Dec, magnitude and band for the standard lines, but for the nonstandard lines with the 300 in columns 56-65 (marked xxx) I don't understand what this means and so far I haven't found a key that explains it.

I can just ignore them, but I'd really rather know what they are in case they are useful! or important.

From here: https://www.minorplanetcenter.net/iau/DASO/Obs/DASO_000559_Obs.txt

00000000011111111112222222222333333333344444444445555555555666666666677777777778
12345678901234567890123456789012345678901234567890123456789012345678901234567890
               DateDateDateDateRARARARARARADecDecDecDecxxxxxxxxxxmm.mmb
2015-005B    tC2017 09 22.18235 01 55 07.45 +12 36 17.4          13.7 RqD0559Y00
2015-005B    tC2017 09 22.18335 01 55 20.26 +12 34 01.7          13.8 RqD0559Y00
2016-038C    tC2017 09 22.16232 01 16 49.19 +13 00 29.0          11.9 RqD0559Y00
2016-038C    tC2017 09 22.16336 01 17 04.66 +12 58 35.4          11.9 RqD0559Y00
2016-038C    tC2017 09 22.16433 01 17 19.05 +12 56 49.9          11.7 RqD0559Y00
2016-055A     V2017 09 21.35632 00 24 08.35 -01 15 45.6          17.0 VqD0559247
2016-055A     v2017 09 21.35632 1 008.0567   +50.9060     300           D0559247
2016-055A     V2017 09 21.35702 00 24 08.04 -01 16 01.2          17.1 VqD0559247
2016-055A     v2017 09 21.35702 1 008.0567   +50.9060     300           D0559247
2016-055A     V2017 09 21.48556 00 23 00.22 -02 01 37.3          16.9 VqD0559247
2016-055A     v2017 09 21.48556 1 008.0567   +50.9060     300           D0559247
2016-055A     V2017 09 21.48625 00 22 59.83 -02 01 53.0          17.4 VqD0559247
2016-055A     v2017 09 21.48625 1 008.0567   +50.9060     300           D0559247
2016-055A     V2017 09 21.48660 00 22 59.53 -02 02 02.0          17.0 VqD0559247
2016-055A     v2017 09 21.48660 1 008.0567   +50.9060     300           D0559247
2016-055A     C2017 09 21.88530 00 23 55.81 -01 22 12.6          17.1 VqD0559595
2016-055A     C2017 09 21.88701 00 23 54.89 -01 22 45.3          16.6 VqD0559595
2016-055A     C2017 09 21.88872 00 23 53.96 -01 23 18.6          16.9 VqD0559595


See also: https://www.minorplanetcenter.net/iau/DASO/Obs/DASO_000562_Obs.txt

and also: https://www.minorplanetcenter.net/iau/DASO/Obs/DASO_000564_Obs.txt
Those are data from "roving observers", essentially any ground-based observation that isn't from an observatory blessed with a three character observatory code. The observatory code is columns 78 to 80 of the entries. Note that the observatory code for those roving observer entries is always 247".

The longitude, latitude, and elevation of the known observatories are known. This isn't the case with those "roving observers", which is why they take two lines. The first line of a "roving observer" entry contains a "V" in column 15 and "247" as the observatory code. The second line of a "roving observer" entry contains a "v" in column 15. The bulk of the second line contains the longitude, latitude, and altitude of the observer.