Notes:

1. Inner loop, or concavity.
2. Concavity.
3. Ambiguous top edge – Curved or flat.
4. Possible left-hand edge.
5. Lower curvature.

A 0 requires a curved top and a curved bottom, and either convex or straight left and right edges. There is minimal support for these features in symbol #18. This excludes a 0.

A 1 needs a narrow width, horizontal top. There is no support for this as symbol #18 has a wide curved top. A 1 needs long vertical edges. There is no support for this in symbol #18. This excludes a 1.

A 2 requires a short edge to the top left, top curvature, and a diagonal from the lower-left to the upper right, and a wide flat bottom. There is minimal support for these features. This excludes a 2.

A 3 requires a short edge to the top left, a concavity on the right-hand edge, and lower curvature. There is minimal support for these features in symbol #18. This excludes a 3.

A 4 needs a left-hand vertical edge, or a diagonal from the middle-left to the upper right, and a vertical right-edge. There is no support for any of these features. This excludes a 4.

A 7 needs a wide horizontal top, and a diagonal from the lower left to the upper right, and a narrow foot in the lower-left. There is no support for 2 of these features. This excludes a 7.

An 8 requires a closed loop on both the top and the bottom (requires upper and lower curvature, and left and right concavities). Symbol #18 has minimal support for these features. This excludes an 8.

A 9 requires a closed loop on the top. A 9 also requires a diagonal line from the lower-left to the upper right, and a narrow, horizontal foot. There is minimal support for this in symbol #18. This excludes a 9.

A 5 needs a wide horizontal top, a top-left corner, and a short left-hand vertical edge from the top, and lower curvature. Symbol #18 supports these features.

A 6 requires a left-hand straight vertical edge, or convex edge. A 6 requires a lower loop with lower curvature, and a right-edge concavity. Note number 4 points out a small vertical section which may differentiate the symbol as a 6 vs. a 5.

Outline: The yellow overlay is the hypothetical outline of symbol #18. This overlay is the partial outline for a “6” which is supported by the evidence.

Conclusion: The symbol features suggest TIGHAR artifact 2-6-S-45 symbol #18 is probably a six (“6”). The most likely first alternate is a five “5.”