@jafant I would be very surprised if your late production SEs did not have the FST boards. Early units might have Lexington boards, Rob Gillum might be the only person who knows.
Read the following knowing that I am an insane person:
If you’re going to upgrade FST boards, my advice is pretty much replace everything. Well, on an extreme budget you might just upgrade the resistors to Mills. But the parts quality is disappointing on the FSTs. The Clarity SAs are good but even those are outpaced by the newer CSAs.
For units with Lexington boards, the parts quality is higher (those Clarity coax feed caps are the best OEM coax feeds regardless of standard or SE version, or Lexington or FST). On a budget, you might just upgrade the resistors and feed caps. Standard CS2.4 owners with Lexington boards could have something notably better than the SE version with just those “simple” changes. Even reaching higher, there are several coils and a cap or three that are worth salvaging. My extreme makeover has 3/5 coils and one bypass cap that are equivalent to Lexington. But I retained nothing whatsoever from the FST boards, not even the hookup wire and binding posts.
