patent pending or patent applied fore. indicates it's one to 3 years, from patent approved.
It depends, I've seen patent take ten years, maybe it was contested or held up by lawsuits. In this water heater's case the author said "The last patent date is 1917,"
Which means the design may be older but that actual appliance in question with the date stamped in the iron or engraved on a plate was made 1917 or later and could have been made in the late 20's or even into the 30's or later before the manufacturer discontinued the model, sold out, merged or went out of business. You wouldn't find the appliance being made in 1910 with a FUTURE date like patented 1917 stamped on it, and those made during the approval process would say "Pat applied for" or pending.
The last patent date only indicates the last time that model/design was altered in a major way, patents cost a lot of money and take a lot of time, and for a simple nothing of utility appliance like a gas water heater- basically a tank of water and a flame this is just not something that's so innovative or high tech it needed protection done on it every couple of years like the telephone did or computer software.
In my line of work a certain wood part with a lead tip on it for wood organ pipes is made by several companies, the part was patented around 1901, and even the new ones made today by one company still carry that patent date stamped on them, so there again is an example where the patent probably long expired but they still put it on more for nostalgia than lawsuit probability since the part is so easy to fabricate a hollow wood tube with a rounded lead tip on one end nailed on, and several companies do.
As far as the water heater goes, I saw at least two yesterday on Ebay, one has a starting price of $9.95, I doubt these are in any way rare if the one time I look on Ebay there are two, but like all these larger items it's local
pick up only or pay $$ for crating and shipping.
