During Interwar Period (1921-1938) Dubno was a county seat > Rowne region. Since your family was likely DP (post WW2 Displaced People), it would be worthwhile for you to make an inquiry thru Bad Arlosen Archives. This would confirm whether your Grandparents were from Dubno, the town, or from one of the many villages in the Dubno district. Immigrants frequently referred to their district center as place of origin for geographic reference, because their own villages were small. Bad Arolsen inquiry by family is available for free.http://www.its-arolsen.org/en/homepage/index.html
Post WW2 and pre Immigrationhttp://www.dpcamps.org/
What religion did your Grandparents practice? Were they Greek Catholic, Roman Catholic or Orthodox?
PS> Maczula - Matschula - Machula is an Eastern Slavic occupational surname, rooted in pagan nickname for person, who produced plant fiber for weaving. Today there are still 1,300+ people in Ukraine, who carry this surname. (This surname shows up as having Germanic origins as well.)