I agree that it seems likely that most of these should be 0s, but another option is that some of them should be 999,9997 or 999,9998. There are a small number of cases with 999,999 values for realprop_1 in the 1850-1880 linked file too (but way fewer than in the 1860-1880 and 1870-1880 linked files, which do not have 0 values for realprop_1 as you note). Thank you for context for clarifying this issue; I will continue to follow up with my colleagues who worked on the linked files to track down an answer (and hopefully documentation) for you to provide to reviewers.