Well, based on the version numbering 1.51 would be newer than 1.101 and iirc 1.51 was a Fast Lane release so, it's not something that would be available to non-Fast Lane people anyway. The OP is just going to have to join the Fast Lane program to get the D-Star capable version(s) it seems.
If they're using <majorVersion#>.<minorVersion#>, 51 would be lower/older than 101. And I see no 1.51 listed in the Fastlane changelog file (notes.txt). The oldest version listed is 1.112 and there is a jump from 1.114 to 2.5 nothing in between)
1.51 sounds like the version that added stuff like LRRP and NXDN talkgroup and radio ID decoding. There were threads about it maybe 18 months ago?
But if the OP is looking for D-Star voice decoding in DSD+, yes, the changelog has that showing up in 1.112
Several UK scanning groups have the same question posted around the same time. On at least one it was answered and with links to a sharing site for download.
To go back to the numbering, 1.51 is not part of the DSDplus/fastlane numbering but one that was left as open source. It got to 1.7x on github and can still be downloaded/compiled (though I'm not sure there would be much point for a non-programmer as it is way behind the DSDPlus/fastlane releases in terms of features and useability..