Pressure-cycling an old cast iron boiler >10psi just plain isn't good for it. Pressure swings that high may not stress a water-tube boiler or mod-con very much, but even with new 50psi-rated cast iron boiler keeping it's operating pressure range withing a 5psi window and running the system at the lowest pressure that doesn't suck air is the right way to go every time. It's not as if there's significant up-charge for going larger. Going with the absolute minimum size that works is poor economy over the lifetime of the system.
Badger's "Lower the pressure until the highest vent sucks air and add 3-5 psi" recommendation works. It defines the minimum operating pressure that still works for the system. With the fill valve shut, if you slowly drain the until the vents a the top just begin to pull in air, the pressure at the boiler will still be something significant. The static pressure at the boiler's gauge will be about 0.43psi per vertical foot between the gauge an the highest vent in the system. If that happens to be 25', it'll be about 11psi. If its 30' it'll be about 13psi. Whatever that turns out to be, that's the minimum pressure it can be with the system cool & idling, with the water at fully cooled idle, say 60-65F (and not 45F water from a super-cold fill.) Adding a few psi from there guarantees that no point will any point in the system be at negative pressure (whether it's hot or cold, pumping or not), and pulling air into the system to add to corrosion, or vapor-locking the system from flowing properly.
With the system buttoned up and filled, with an expansion tank properly sized and charged, if it continues to lose pressure over the course of a week it's losing water somewhere. It would not be surprising on a boiler that age to have weeping at the plate seams when the system is cold (especially if it's had 57 years of 15psi+ pressure cycling.) As long as it's not puddling a properly operating auto-fill valve would mask the symptom and get you through the season. But whether it's leaking or not, swapping in a newer more efficient boiler in the intermediate-term is going to be economic.
The high mass of the system means that it would never short cycle even if the new boiler was oversized, but it's still a good idea to right-size the replacement boiler, and it'll almost certainly have lower output than this one. In 1955 systems were typically at least 2x oversized for the heat load, even at the less insulated less tight and single-paned windows of the day. If there have been any improvements on insulation, air tightness and windows since the system was installed it could easily be more than 3x oversized. A careful room-by-room Manual-J type heat load calculation based on the construction type and exterior surface areas, etc, &/or using the billing to measure fuel use against heating degree-day data between the meter-reading dates could put some hard upper bounds on the boiler size. Oversized boilers cost more up front, and deliver less comfort, even when the mass of the system is keeping it from falling off an efficiency cliff by short-cycling.