I believe this renovation plan for Lincoln Park is likely the culprit for some sources stating that the Union Grounds were on the eastern side of Hoeffer (Dalton Ave today) and directly adjacent to Lincoln Park.
This is from the Lincoln Park Grounds Wikipedia entry:
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Lincoln Park was bounded by Kenner Street (north); Freeman Avenue (east); Hopkins Street (south); and Hoefer Street (later Dalton Street) (west). Old maps show the western one-third of the park designated as "ball field".
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Source:
Lincoln Park Grounds
The article appears to be describing this plan exactly even though the "Ball Field" area clearly looks like a soccer/football field and nothing like any depictions of the Union Grounds.