So with Bluestreak being WFC-S64, I have a few thoughts (read rampant speculation) on the missing numbers.
My guess is that most of the numbers above S51 will be either 35th anniversary ones or SDCC/online/store exclusives that weren't finalized before Hasbro put together the map with the codes for the regular Siege releases and the Amazon exclusives we know about so far. If that is correct, then 44-47 should be another wave of new micro and battle masters.
With Soundblaster coming in Siege, it should be almost certain that we will get at least 1 more pack of cassettes if not more. I think there is a decent chance of getting a pair in S44/S45 and the 35th anniversary releases, especially in the 35th anniversary. Why sell a Soundblaster with room for chest minions and not sell some minions at the same time?
So far the regular releases were numbered in increasing order for the same wave. If Bluestreak, Soundwave, Prime and Megs are all getting released together, then wouldn't it be possible that the voyagers are actually S65-S67?
So, assuming S44-47 are another wave of masters, then we have from S52 to S60 on the low end and S63 on the high end depending on where Soundblaster/Prime/Megs fall. That's 9-12 unknowns aside from S44-47.
Let the speculation begin!!!! Personally I think Sunstreaker and Wheeljack are good bets since they appeared on the posters/packaging already.
And of course now I'm going to run down to the basement and obsess over all the Siege packaging I have so far.