My guess would be Mosaic wanted to feature the Getz-Raney unit as a working group. The sessions before 1951 would not match that.
There was more recorded at that 1948 session for Sittin' In With:
------------------------------------------------------------------------
Stan Getz Quintet : Stan Getz (ts) Al Haig (p) Jimmy Raney (g) Clyde Lombardi (b) Charlie Perry (d)
New York, October 25-26, 1948
C121-1 Pardon my bop (*) Dale LP21, EP201, Spotlite (E)SPJ140, Mainstream MDCD722 [CD]
C121-2 Pardon my bop (*) Dale LP21, EP201, Spotlite (E)SPJ140, Mainstream MDCD722 [CD]
C121-3 Pardon my bop SIW 532, Jade 702, Vogue (E)V2030, EPV1016, Jazz Sel (F)630, Spotlite (E)SPJ140, Definitive (And)DRCD11257 [CD]
C122 As I live and I bop [Bopcycle]SIW 505, Mainstream M56025, Jax 5002, Dale LP21, Mainstream MRL364, MSTD101, Spotlite (E)SPJ140, Mainstream MDCD722 [CD], Definitive DRCD11257 [CD]
As I live and I bop (alt,*) Mainstream MDCD722 [CD]
C123 Interlude in Bebop [Bopelbath] (*) SIW 549, Dale LP21, EP201, Jade 702, Vog (E)V2031 Spotlite (E)SPJ140
C123 Interlude in Bebop SIW 505, Dale LP21, EP200, Vog (E)V2033, EPV1016, (Dan)JS673, Jazz Sel (F)673, Mainsteam M56025, S6025, MSTD101, MRL364, Spotlite (E)SPJ140, Dejavu (It)DVRECD17 [CD], Mainstream MDCD722 [CD]
C124 Diaper pin [Pinhead] (jr out) SIW 532, Dale EP200, LP21, Vog (E)V2031, Spotlite (E)SPJ140, Mainstream MRL5007, MDCD722 [CD]
C124 Diaper pin (jr out) SIW 549, Dale LP21, EP201, Vog (E)V3031, EPV1016, JS630, Mainstream MRL5007, MRL364, Spotlite (E)SPJ140, Dejavu (It)DVRECD17 [CD], Mainstream MDCD722 [CD]
Note: Mainstream MRL364 titled "Yesterday"; rest of this LP by Wardell Gray, Al Haig and others.
Mainstream MDCD722 [CD] titled "A look at yesterday"; the rest of this CD by Wardell Gray, Paul Quinichette and John Hardee.
All titles, except (*), also on Classics (F)1126 [CD].
One take of each of C122, C123 & C124 also on Mainstream (E)MRL5007.
One take of Mx. C123 & Mx. C124 also on Definitive DRCD11257 [CD] titled "Stan Getz & Jimmy Raney - Complete Studio Sessions"; a 2 CD set.
One take of each title also on American Jazz Classics (Sp)99091 [CD] titled "Stan Getz Quartet & Quintet - The Complete Roost Studio Sessions"; a 2 CD set.
One take of each title also on Definitive (And)DRCD11136 [CD], Blue Moon (Sp)BMCD99958 [CD].
------------------------------------------------------------------------------------------
A discographical mess indeed. If I read these data correctly, no reissue ever included all takes.
There were sessions in 1949 with Getz and Raney for Savoy, Seeco, and Harry Lim, too. Kind of a pool of musicians.