Stephen Percy Larcombe
John, I think you are wrong. There is such a thing as an "egg ended boiler".
This boiler looks pretty much exactly like what they should look like.
The book "Historic Steam Boiler Explosions" by Alan McEwen has a write up on a bank of these boilers exploding at a blast furnace.
The boilers were described as 64 feet long, externally fired (hence no fire tubes or furnace tube(s)). They were fired on blast furnace gas. They ran at about 54 psi (if my memory is correct). Boiler number 5 failed, the resulting release of steam and movement of the boiler upset the brick piers that the other boilers were mounted on, and resulted in a chain reaction, that caused a total of 12 boilers to explode.
These boilers all had single rivetted lap seems. The pictures in the book match the online pictures. Remember also, that this explosion occured in the 1880's when the boilers were somthing like 30 years old. So do not expect too many modern safety fittings. That also gives plenty of time for the boilers to have found a second use after the steam generation part of thier life has finished.