This one is as simple as Backport dar, except for the strange version:
As explained at , version
+b1 means "1st recompilation", ie the package was recompiled without any change in the source code (but probably with a different build environment).
1: is a version prefix, usually because of a non-incremental version change , we don't care about it.
So the source version actually is
1:0.7.3-1, and hence the bpo version will be
Note: in the backports archive, the package is called differently: this is because we used a different versioning convention at that time. You can ignore this discrepency.