This "6.Batal. K.O.P." can be Batalion KOP "Iwieniec"
[KOP Battalion "Iwieniec"] (created as 6 Batalion Graniczny,[Border Battalion] later renamed to 6 Batalion Graniczny "Iwieniec").
Please see HERE
for additional information in Polish.
Most relevant information from above source would be that:
- between Sept. 4. and 6. 1939, the battalion mobilised (in 2nd wave of general mobilisation) III batalion 207 Pułku Piechoty (Rezerwowego) [III battalion of 207 Infantry Regiment (Reserve)]; it was commanded by Major Jan Styliński; it took part in defence of Lwów [Lviv, Lemberg] as a part of 35 Infantry Division,
- when mobilisation of III/207 pp has been completed, new reservists were pressed into its ranks and the battalion was recreated; the battalion has been subordinated to commander of Pułk KOP "Wilejka" [KOP Regiment "Wilejka"]; OdeB of recreated Batalion KOP "Iwieniec": battalion's headquarters, reserve company, HMG company, signal platoon, 1 Kompania "Żebrowszczyzna" with reserve platoon and 3 sentry posts/watchtowers: "Zagajno", "Wołma" and "Joachimowo", 2 Kompania "Raków"with reserve platoon and 2 sentry posts/watchtowers: "Kuczkuny" i "Pomorszczyzna"; 17.09.1939, after Soviet invasion, the Batalion KOP "Iwieniec" took part in several-hours-lasting fights with units of Soviet 6th Cavalry Division, as a aresult of which the battalion was destroyed; kapitan Edward Michał Nowrat - battalion's commander - and his deputy were shot/executed on the spot after they surrendered their battalion south of Lida; one of battalion's bigger fights was held against Soviet 145th Cavalry Regiment in area of Żukowo-Borek village at Niemen river.
Now, if your Dad was captured on 3rd day - this would mean September 20th - then it would be too early for surrendering Lwów to Soviets (22.09.1939), and too late for border fights. More in-depth investigation is needed
"Always be ready to speak your mind and a base man will avoid you" W. Blake, Proverbs of Hell