Will expand this later, check in FNCT and other sources. I have never seen this species before, so take this information tentatively.
S. drummondii and S. parvula can appear to have similarly sized corollas, and both have longer white hairs on the calyx. Normally it appears that the size of the corolla relative to the calyx is smaller when looking at S. parvula, but other characteristics should be used to verify this species.
The length of the flowering pedicel relative to the calyx
For S. parvula, I've noticed that the flowering pedicel is much more prominent than that of S drummondii. The length of the flowering pedicel is about the same as the length of the calyx. On S. drummondii, the pedicel is inconspicuous and the flowers appear almost to be sessile.
The molted speckles on the corolla
The dark speckles or dots on S. drummondii appear to remain restricted to the white part in the middle. For S. parvula these speckles will more often than not reach outward to the edge of the two front corolla lobes.
The shape of the leaf base
According to Flora of North Central Texas (pg 778-779), S parvula can be distinguished via the leaf base, which are "subcordate to truncate" compared to "rounded for tapering bases" for S. drummondii (and S. wrightii)
Petiole (or, in this case, lack thereof)
S. parvula has sessile leaves, S. drummondii has petioled leaves, albeit winged petioles (leaf tapers and extends along the petiole to the stem).
https://www.inaturalist.org/observations/123925798
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