According to this page, Fire-Lite had CP351 and SD351 detectors in the 1990s in addition to the CP300 and SD300, but no further information is provided on the website. Based on this UL listing and this UL listing, the detectors appear to be plug-in and compatible with B301LP and B350LP bases. I have never heard of the B301LP before.
Also, note that the UL listings mention the SD351T, which is not explicitly mentioned on Fire-Lite’s website, although it could be part of the SD351 listing. This might mean the 351 series detectors are rebranded 200 series detectors, since the 2251T was not introduced until 1999/2000, but if that was the case, I do not see why Fire-Lite would not show the products on the website. I am also not sure how these would have been different from the 350 series detectors, besides possibly not including a B350LP in shipping.
The UL listings also mention an SDRF-351 and SDRF-351S. These are probably similar to the Notifier SDRF-751, which might be rebranded from Ademco. If this was the case, it would make sense for the CP351 and SD351 to be from the 200 series.
The devices listed above are smoke and heat detectors, and the 2120 is a panel, I don’t see why the 2120 is better if they are different devices on a FA system.
Ademco produced the 5808, which appears to be a wireless version of the 2300T/2300TB. Prior to the release of the MS-9200, Fire-Lite produced the MS-4812 and MS-4824, which had loop cards for Ademco SLCs. It is possible that these systems also had a wireless interface module to connect to wireless Ademco devices and the SDRF-351 was a rebranded 5808 for these systems. As far as I know, Ademco never produced a wireless version of the 400 series.
System Sensor did not introduce Direct-Wire 100 series devices until 1996, which meant the 400 series devices were the only Direct-Wire devices when the MS-9200 was introduced in 1994. It is possible that the CP351 and SD351 devices were plug-in 200 series devices, but the 300 series devices were based on the 400 series devices because there were no low profile Direct-Wire detectors.
The listings mention the SD351 and SD351T being capable of mounting to the B301LP and B350LP. A standard 200 series detector could be mounted on a B501 or B210LP, the latter being rebranded as the B350LP. There is also a B310LP that appears to be completely identical to the B350LP, and it may have been the old name for the base while the 351 series was in production. It would also make sense to refer to the B501 as a low profile base because the B501 is a flangeless base.
Since the linked B310LP manual is from after the CP350/SD350 were introduced, Fire-Lite may have updated the 351 series at this time as well. At this time, the 351 models may have referred to the detector head subassembly in 350 series devices, which would explain the purpose of the SD351T.
The CPID and SDID (rebranded Ademco 4192CP/4192SD) used with the MS-4812/4824 were Direct-Wire devices using screw terminals, similar to the 400 series. The 300 series achieves a middle ground between Direct-Wire and plug-in using a removable terminal block, similar to the 2300 series, which means the 351 series may have been the predecessor to the 300 series.
Both listings mention air velocity, but this does not seem to be reliable. For example, the CP451 (System Sensor 1451) is listed as having a maximum air velocity of 300 fpm, while other datasheets indicate that the 1451 had a maximum air velocity of 1200 fpm. I have not found any System Sensor detectors with a maximum air velocity of 300 fpm.
Most of this seems to suggest that the 351 series was a series of plug-in 200 series devices.