the $\{ 4-3 \}$ interval, we obtain an average value of 201.260(20) MHz from a set of 10 measurements with a standard deviation of 33 kHz. The value from the work in Ref. [@TAW88] was 201.240(20) MHz, while the more recent frequency-comb work in Ref. [@GDT03] reported a value of 201.287(1) MHz. The inconsistency of $2.4 \sigma$ is smaller but still quite significant. Again, our previous result of 201.266(6) MHz obtained with AOM locking [@DAN05] overlapped with the earlier value but was inconsistent with the frequency-comb result. Our new value, though with a larger error bar, gives confidence in the previous measurement.

Conclusions
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In summary, we have shown that hyperfine spectroscopy with co-propagating beams in a vapor cell has several advantages over conventional saturated-absorption spectroscopy. In addition to the usual mechanisms responsible for probe transparency, there are EIT effects that enhance the peaks, which is supported by density-matrix calculations. As a result, the primary peaks are more prominent and appear with good signal-to-noise ratio. The transmitted signal appears on a flat background (Doppler-free) and does not have the problem of crossover resonances in between hyperfine transitions (which are stronger and often swamp the true peaks). Absorption by non-zero velocity 