I remember you mentioning that system, and how part of it had TrueAlert speaker/strobes and 4903 horizontal speaker/strobes. I think the latter signals replaced older Federal Signal speakers and 2903 lights, and the 4251-30 pulls you mentioned are most likely the addressable version. And I recall quite a few Simplex technicians online saying that Simplex 2120 systems are a hassle to work with and are still supported by the company. Here’s a video a technician recorded and uploaded onto MySpace of a Simplex voice-evac system that had a 2120 replaced by a 4100U. The old Simplex LifeAlarm speaker/lights are still intact, and the 4251 pulls are too. Pretty neat seeing the 4100U slow whoop/voice message come out of a LifeAlarm speaker/2903 light setup…
It reminds me a lot of the system at Brockton Hospital, which most likely also has a Simplex 4100U panel, and they have many of the old LifeAlarm speakers and 2903 plates still intact, and the 4251-20 pulls. Many of the old smokes have been replaced with TrueAlarms, though.
Hopefully that hospital you mentioned will be able to replace that 2120 with a 4100U sometime in the future. They wouldn’t even need to replace the existing signals or pulls! I’m pretty sure addressable 4251 pulls are fully compatible with 4100Us.