The "X-Plus" suffix to the Steelfish name was used in 2006 to distinguish the new, larger (44mm) Steelfish from the older 42mm version. The following year, the X-Plus was dropped from the name. There's no difference in an X-Plus A17390 Steelfish to a non X-Plus A17390.
A quick way to tell the rough age is if the watch has a square or arrow head seconds hand. The arrow head first appeared in the 2008 Chronolog so would indicate a late 2007 or later manufacture.
You can find out precisely when your watch was manufactured by looking at the tiny numbers between the lugs at 6. The 4 digit code is week number/year so 1508 would be the 15th week of 2008.
Hope this helps.
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