Documents often contain multiple pieces of information potentially given by multiple informants. Even one informant can be knowledgeable about some things and not about others. Just because one piece of information on a document is wrong does not mean they all are incorrect. Just because they got the first nine pieces of information correct (that you’ve checked) does not automatically mean the tenth piece of information is correct. There’s always shades in terms of how much is accurate and one has to look at each piece of information separately based upon who the likely informant was and how likely they were to know it. I’ve been reminded of this while working on a Civil War veteran who gave two significantly different sets of family information in his pension […]
Ultimo means “previous” (usually month). Instant means “this” or “current” (usually month). When the Bureau of Pensions wrote on 4 January and said “Your letter of 15th ultimo,” they were referring to a letter of 15 December. When they wrote on 10 December and said “Your letter of the 4th instant,” they meant the 4th of December.  
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