The following information is courtesy of Roger Burdette.
The 1921 high relief Peace dollar design was subject to poorly defined strikes and rim "fin." Between January 2 and 23, 1922 US Mint Engraver George Morgan attempted to correct these problems by remodeling the coin to reduce
the relief and sharpen details. The 1922 medium relief Peace dollar is one of what may have been several experimental designs made by Morgan with the approval of Mint Director Baker and sculptor James Fraser. All of Morgan's design attempts were failures. In late January, frustrated with the Mint's inability to reduce relief of the 1921 design, deFrancsici and Fraser agreed to preparation of new models in low relief for the 1922 coins. These were completed in early February. This produced the low relief 1922 design used for circulation coins.
The reverse of the medium relief 1922 Peace dollar is from the same hub as the rejected 1922 high relief sandblasted proof Peace dollars (Norweb, and others) which is actually also a pattern. See J2019 for more information.
3,200 medium relief pieces were produced. Three examples were sent to the Mint Director: 1-sandblast proof, 1-satin proof, 1-normal strike. These same coins were also sent to Fraser for examination. The satin proof and the 3200th were in the estate of Mint Director Raymond T Baker and offered by Stacksbowers in the August 2014 ANA sale.
The following 6 examples are presently known.
1) Bowers & Merena 9/85, lot 652 - Choice Proof. This was not recognized as a medium relief at the time.
2) The Discovery Specimen - NGCAU55, illustrated above courtesy of NGC.
3) An example certified as NGC64.
4) Raymond T. Baker Estate, StacksBowers 8/14 ANA - PCGS Satin PR67, illustrated below courtesy of PCGS
5) Raymond T. Baker Estate, StacksBowers 8/14 ANA, the 3200th piece struck - PCGSMS65, illustrated below courtesy of PCGS
6) A piece slabbed NGCAU50 in June 2021