The correlation between ship date and serial numbers for this model are pretty much in order, so estimating ship dates is possible. 219935 was shipped in an era where numbers sold were very low. It took 5 years to sell 10,000 between 210XXX and 220XXX , but still in order in the database, all from 1916 to 1921. 181,621 would most likely have shipped around 1911, indicating that sales between 1911 and 1916 totaled near 30,000.