Figured it out. It's a format used by OTH-Gold, as described in Appendix C here:
https://apps.dtic.mil/dtic/tr/fulltext/u2/a405953.pdf
Enter the position of the report in its original format and precision if possible. Use one of the alternate field contents provided below. Enter the designated field descriptor followed by the data. Data can be expressed in:
Coordinate System Field Descriptor
LL: Latitude/Longitude
UT: UTM (Universal Transverse Mercator)
GR: GEOREF (World Geographic Reference System)
The precision reported in this field should reflect the original known precision. The only boundary or restriction placed on the reported precision of data in this field is the field length range. An optional floating decimal is allowed as appropriate, e.g., LL:304055.55N7-1304055.55E8, UT:45FDK0474, GR:DIQA