10.50 was Windows-only too (the day after the brower ballot screen went live, 2 March 2010); Mac/Linux not catching up till 10.60 (1 July 2010).
(Note also that there had been large delays in Presto reaching Desktop before — most notably with Opera 10 (2.2.115) and 10.10 (identical but with Unite enabled), Mobile shipped a beta of Mobile 10 with Presto 2.3.something before 10.10's release, and a second beta with 2.4.15 shortly after. Note from late 2.3 Presto releases, the final digit is the most significant one.)
(Note also that there had been large delays in Presto reaching Desktop before — most notably with Opera 10 (2.2.115) and 10.10 (identical but with Unite enabled), Mobile shipped a beta of Mobile 10 with Presto 2.3.something before 10.10's release, and a second beta with 2.4.15 shortly after. Note from late 2.3 Presto releases, the final digit is the most significant one.)