Dulebes were a confederation of many Slavic tribes. I would except the name Buzhans/Volynians. Buzhans settled western Bug river.

By "western Bug" you mean west side of Bug (Poland)?

There should be some Polish literature on archaelogical findings in your area, which could shed some light about the archaeological cultures in your area.

I was searching for it (as did some people from this historical forum) and we found nothing. In general about Buzhans/Volhynians we can find only something, which is connected with more southern part – Busk, Gródek. While Busk lies in Ukraine (Lviv Oblast' if I remember correctly) and Gródek is in Poland, near the border with Ukraine, in the middle of Lublin Voivodeship. I live on the northern part of it.

What else I've found are three places from these times. It is Zbucz, Zajączki and Klukowicze. There is something written that we can connect it with "Mazovian wave". I don't know if I can understand "Mazovians" by it anyway. Klukowicze are located on the southern part of Podlaskie Voivodeship, Zbucz is a bit on north-east from Klukowicze and Zajączki almost in the middle of the voivodeship. These "grods" were destroyed before 1041 by Kievan dukes.

So Klukowicze are the closest place from my place. The distance is 101km, while Gródek is 180-190km.

But the earliest information (except neolitic times) about exactly the closest cities comes from:

1. Międzyrzec Podlaski (13km) – 1174, a date from church document, a first chapel on this area, probably Orthodox of st. Nicolas; 1369, first time is mentioned in 13th-century Ruthenian chronicle speaking about 10 villages located around Krzna river; 1390, Abraham Chamiec earned Międzyrzec from Władysław Jagiełło; 14-15th centuries, a development of the settlement connected with a colonisation this place by the Mazovians. As "Międzyrzec" it was firstly mentioned in 1390.

2. Biała Podlaska (25km) – 1345, first owners are Iliniczowie, Lithuanian family. There is believed that the founder of a city is Piotr Janowicz "Biały". Anyway – to late times.

3. Radzyń Podlaski (43km) – almost to the end of 14th century the territory was located near the borders with Lithuanian-Ruthenian country and Yotvingia, so settling here was to risky. After taking a throne by Władysław Jagiełło the situation had changed. After 1385 people from Mazovia and Polonia Minor began to settle here. On this area was a place where have met Mazovia, Polonia Minor, Duchy of Lithuania, and Ruthenia.

4. Łuków (48km) – first time mentioned in 1233, there was a castellany which belonged to Polonia Minor that has strategic meaning, located near the borderline. In the middle of 13th century a prince Bolesław Wstydliwy put Templars in Łuków. The city was multiple times destroyed by the Old Prussians, Tatars, Lithuanians and Yotvingans. Polish-Lithuanian union in 1385 calmed the situation.

5. Siedlce (54km) – emerged in 15th, so it doesn't matter.

6. Brest (70km) – as Zbucz, Zajączki and Klukowicze is connected with this "Mazovian wave". One of the oldest settlements in Polesia.

So it could be that Mazovians lived here. In Radzyń lived no one at these times, but there is mentioned it was close to Yotivingians. In Międzyrzec we know only that in 12th century there was something mentioned about Orthodox Church and Mazovians centuries later. About Biała we know nothing, and Łuków and Siedlce are not important. Siedlce are in Mazovian Voivodeship, Brest – as you know – is in Belarus.

So could be them. But these information are based on the Christian era on these lands, and I am interested about pre-Christian times, circa 10th century. It is a fact that Mazovians built these cities in the 10th century however. But there is no info about their "southern borderline". And Mazovians described in Międzyrzec text are not a tribe anymore, but a people from Mazovia – one of Polish districts.