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M2 and M1

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1910 U.S. Census, Canton Borough, Bradford County, Pennsylvania, enumeration district 10, sheet 2B
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Williams, Eliza | Grandmother | F | W | 78 | Wd | | 4 | 2
McBride, Henry C. | Head | M | W | 79 | M2 | 25
- Mary | Wife wf | F | W | 71 | M1 | 25 | 0 | 0
Fellows - Myron | Head | M | W | 83 | M1 | 61
- , Adeline | Wife wf | F | W | 80 | M1 | 61 | 6 | 5
Stone, Justin W. | Head | M | W | 64 | M2 | 19
- Emily | Wife wf | F | W | 62 | M2 | 19 | 2 | 1
Source: Pennsylvania- BRADFORD County, Enumeration District 10, Sheet No. 2B (DPLA 3eaf1774ec3f99845cd9cbc178c55e86), Department of Commerce and Labor, Bureau of the Census; National Archives and Records Administration. Public domain.
Henry is marked "M2" and Mary "M1", both with 25 years of marriage. What does that tell you?