Yep. Parkerizing became popular during WWII.
I did some research a while back that was enlightening. I learned that parkerizing is actually a "sponge" that absorbs and hold liquid, be it oil or water (!). That's why it's important to keep parked guns oiled; if the matrix is not filled with oil, water will be absorbed and held against the metal. Properly cared for, a parked surface is as rust-resistant as they come.
While inexpensive, it is NOT "cheap"! In fact, it is used as part of "more modern" finishing processes.
i STILL don't know what the ACTUAL government-wide "changeover date" was as my 1937-38 "SAAA rebuild" (for the British Home Guard) on a M1917 (P-17) rifle is sandblast blued OR a finish that LOOKS like bluing.
(i've wondered if the finish is bluing OVER Parkerizing, but just don't know.)
I don't believe you can blue over parkerizing. Bluing was rare in the US Army of the 1970s, but the shop I worked at at Ft. Hood, Post Maintenance had both capabilities. Getting the surface perfectly clean just before it went in the vat was the goal. 90mm Recoiless Rifles were blued, some Model 10 S&Ws were blued, everything else and some badly packaged WWII surplus parts were parkerized.
Who prefers Stainless Steel finished black. SWaMPy 9c.