Measurements, performed at NBS, NPL, and PTB, on helium-neon lasers stabilized on the Lamb dip, have shown that the wavelengths of these lasers fell within approximately 1 part in 107. Beyond this limit, different lasers were found to emit different wavelengths. In addition, the wavelength of a given laser may vary during the life of its discharge tube. Pressure shifts appear to be a major cause of these variations.
K. D. Mielenz, K. F. Nefflen, W. R. C. Rowley, D. C. Wilson, and E. Engelhard, "Reproducibility of Helium—Neon Laser Wavelengths at 633 nm," Appl. Opt. 7, 289-293 (1968)