Windows Server 2003/5.2.3790.1711.dnsrv r2 beta.041203-1815
From BetaArchive Wiki
Most probably the first R2 build leaked through Connect. Known to exist in x86/x64 isos. The ISOs from MS Connect were also released by WiNBETA. This possibly came with Server 2003 SP1 build 3790.1289.
Therefore this is a lost build.