Thanks Raven.

All I can do is scratch my head at this point and hope some of this absorbs.

1246 GYIPE APRDGQAYVR L KDGEWVLLST FLGhhhhhhh

I did bing that difference (end portion of the 'Seattle' 6VXX_A Spike) and curiously the search engine points to a patent that has a vaguely similar sequence in it:

"The codons of CHA5 were optimized for expression by using human codons. As shown in Table 1, the original viral protease cleavage site PQRERRRKKRG (SEQ ID NO: 1) was mutated to PQRERG (SEQ ID NO: 2) in order to prevent proteins from the enzymatic cleavage to form HA1 and HA2. The transmembrane region (residues: 533-555) was replaced with the additional residues"

(SEQ ID NO: 3) LVPRGSPGSGYIPEAPRDGQAYVR KDGEWVLLSTFLGHHHHHH

"at the C terminus of the HA construct, where the thrombin cleavage site is in italics, the bacteriophage T4 fibritin foldon trimerization sequence is underlined" (GYIPEAPRDGQAYVRKDGEWVLLSTFLG), "and the His-tag is in bold (Stevens J, et al. (2006) Science 312:404-410)".

Thus is seems that the Seattle _A sequence has added the "bacteriophage T4 fibritin foldon trimerization sequence".

In summary, it seems that both the start and the tail of the Seattle strain has mystery additions, which is a total mystery to me at this point.