Cumulative distributions of adjacent period ratios. Simulated systems in Scenario IN[C] at 50 Myr (solid lines) with detection probabilities associated with TESS higher than 0.9 (see Fig. 8). Observed exoplanetary systems around stars with masses 0.08 < M_*/M_⊙ < 0.6 (top panel) and around stars with masses 0.08 < M_*/M_⊙ < 0.35 (bottom panel) that host a USP planet (dashed lines). Observed USP-neighbor pairs are compared to simulated USP + candidate neighbors (blue lines). Other adjacent pairs around M dwarfs are compared to all other adjacent pairs in our simulations (orange lines). Vertical dotted lines mark common MMRs: 4:3, 3:2, 2:1, and 3:1. Poissonian errors for each distribution are shown as shadow areas. The comparison tests whether the early-epoch orbital spacing of prospective USPs resembles the presentday spacing observed for mature USP systems.