You have two totally different units there. The vessel (what you call a boiler) originally had no relation to the fire box below. It was made long before the fire box. I am guessing the fire box originally was just a manual wood fired boiler. Their should be a pipe bundle or coil inside hooked to those two pipe stubs at the bottom. They were capped when it was not used as a boiler any more when the top vessel(what you call a boiler) was set on top.. You didn't show what the firebox flue system looks like. The plates on the units are the manufacturer's nameplate and test stamp. The H should be inside the symbol that looks like a 4 leaf clover. That is the National Board of Boiler and Pressure Vessel inspector's stamp. Pebbco should have a record of the vessel and what it was originally made for. Hard telling what it was. Can you tell if those small pipe tapings on the side are the in and out of a coil inside. It may be a side arm heater. Both are long out of date for re-testing. If Waterloo or whoever may have bought them out is still around they should have those records also. Setting the vessel on top like that does not provide good heat transfer so it would be very inefficient. With out an insulated outer jacket you have one large radiant heater. They both were formerly insulated I am sure.
Bottom line: You have a pile of scrap iron you paid $20 for. To make something usable would not be cheap and it will still be of iffy from a boiler safety standpoint. If you have never seen the destructive force of just a small home hot water heater going up in the air or sideways; take it from me you do not want you or your family to be anywhere close. I have seen the results of a relatively small boiler taking a whole industrial concrete block building to the ground.
Boilers are not something for the novice or DYIer should be messing with. Boilers go through a very rigorous manufacturing, testing, and re-testing program to keep them safe. National Board inspectors have a long torturous qualification process. Certified welders and even the shop also have rigorous qualification processes.
Haul it to the scrap yard, Ron
Bottom line