It seems to me that Shanks uses at least 2 swords which he switches occasionally. Because the design changes are to frequent.
In chapter 434, Shanks' sword has one design (A)
In chapter 552 (Ace flashback) Shanks' sword is designed differently (B)
In chapter 579, Shanks' sword is back to design (A)
In chapter chapter 838 (post TS) in the cover, Shanks sword is yet again changed into design (B)
Doesn't make much sense to me to change it back and forth multiple times within such a short time span. When the weapons of others (Whitebeard, Mihawk, etc.) have been exactly the same. Always.