Some time after the conclusion of the F-1 engine test program, some sort of structure was erected in the field between the F-1 Engine Test Stand and its observation bunker.

When I visited the F-1 observation bunker in 2013, I noticed that the area to the south of the bunker was littered with concrete pads (Google Maps link) and wondered what they were. Although I was unable to find any reference to this structure in the Historic Assessment of Marshall Space Flight Center, one MSFC engineer believes that the piping and structure were associated with a composite tank test that was performed.

Building 4696-1: Earth-mounded observation bunker, looking northwest [middleground right]. 11 January 2002. K. Weitze
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From Appendix A page 124 (p. 279 of the PDF) of the Historic Assessment of Marshall Space Flight Center.
Extraction and cleanup by heroicrelics.

Note the F-1 Engine Test Stand observation bunker (Building 4696-1) on the mound at right.