Many options and I don't really know, but here are my opinions: Could be Tripolye-Cucuteni, could be steppe-related. In any case we have R1b and J2b together in Mokrin EBA, preceding the Iron Age expansion of Channelled Ware. So one path could be that it was picked up by steppe or directly Yamnaya related groups already at the border of the steppe in the Caucasus or Carpathians, or later from Lengyel-Sopot or Baden related groups, unified in the EBA of Pannonia-Balkans, was later expanding South and did survive there among Illyrian in particular, even spread along the Pannonian-Eastern Hallstatt route, sometimes together with E-V13, even West, into Western Mediterranean and Celtic territories. In any case, I guess its currently still the first find in Mokrin where R1b-Z2103 and J2b being found together in the Central to Western Balkans. The question is rather did they come together (Caucasus/Carpathians -> Balkan) or separately (picked up by steppe groups). That's not really known at this point if I'm not mistaken.
They (J-L283) did in any case play and important role in the Balkan Tumulus Culture groups, even those I wrote about with clear North Western influences. How they managed to achieve that, from which source group, is also open to debate. I guess simply from the Southern Pannonian sphere, but we need to find out. Glasinac-Mati is in any case the Illyrian core group, contrary to the South Eastern Urnfield/Gava derived groups to their North and East, which were mostly Daco-Thracian, and those people to their North and North West, which were Pannonians, Veneti and Celts respectively. You have to imagine that from the EBA to the IA everything was pushed South, rather. So the people which lived at the Middle Danube and Northern Serbia might very well have ended up at the Adriatic coast in the LBA and that's what we see.
Others might know better in detail of course, especially about the finds and subclades of J-L283.
Mokrin is in any case very interesting because its being dominated by R1b, I2 and J2b just about 500 years before Channelled Ware, with zero E1b1b at all, yet alone E-V13. While with Gava/Channelled Ware and Psenichevo-Basarabi in particular, we can be pretty sure they had 50-100 percent E-V13 and some of the earlier lineages seem to have just melted away. If being found later, they are more likely to have come back, from the areas the Illyrians could hold or kind of "reconquered" later, even if it were different tribes under completely different circumstances. The forth and back of the tide, one time for an Illyrian, the other time a Daco-Thracian or Greek tribe, is what made the region more complex and mixed later and did create the modern people of the region, with new migrants, especially Germanics and Slavs added. But originally, it seems to have been much more clear cut, with some transitional regions early on, like along the Middle Danube-Tisza. That's the image I described for the later periods too, with the Thraco-Cimmerian horizon and Basarabi-Eastern Hallstatt connecting via the mixed Pannonian zone the Alpine with the Carpathian-Balkan region. But the origin of the groups, while distantly related, was different.
By the way, from the 2nd paper quoted before:
https://library.oapen.org/handle/20.500.12657/33895
Things get mixed up at this point, but those still or newly cremating, could deviate. In any case some of the major groups should yield at least enough samples to make it clear, especially Kalakača (E-V13 dominated, mixed?), Basarabi (mainly E-V13 + minor R1a+R1b?) and
Glasinac (J-L283+R-Z2103?).
