From my research on GT350s and Cobras.
4B2 was the first "week" of production for the R-3259 model. Shelby American had some earlier ones but they were R-2668s reworked to R-3259 specifications by Holley for Shelby. The date of rework is unknown but probably in time for early GT350 program testing.
4B5 was the second production date for the R-3259. (Just for fun there was a run of 1964˝ Ford 427 C4AF-9510-DA A revision R-2919 Holley® made the week before for Shelby and one got mounted on cut back door car CSX2513.)
I use to work on a lot of R-3259/R-3259-1 models for people. I noticed that individual parts changed from time to time, example more than one main body casting was used for R-3259 production assemblies. By the time the 1966 Holley specification sheet was sent out there was the prototypes for SAI and four engineering versions as far as Holley was concerned.
R-3259 (R-2668 models reworked to the new R-3259 specifications)
R-3259A (based on the December 2, 1964 dated Holley engineering file specifications)
I have found nothing Holley later than 1966 to add any more information. I did start a table of key parts but haven’t gotten very far.
As far back as the 1970s people have asked me what the differences in versions were. The changes in R-3259 between the prototype and last production version were small changes, including meter block assembly changes, to improve the air/fuel ratio curve throughout the range of operation. The introduction of the R-3259-1 was a fairly significant change level. With the introduction of the R-3259-1 a killer air bleed was added to the vacuum secondary circuit. The design change had two purposes: 1) the bleed reduced or “killed” the vacuum signal for a short period during wide open throttle operation to allow engine rpm to get higher before the secondaries would start opening and 2) add the vacuum signal once sufficient air flow through the secondaries was established to make the secondaries get to full open sooner. Said another way the new precision bored passage slowed the start of secondary operation and then made full open happen faster.