The earliest I've personally recorded is #19932, Grade stamped "4" and both tubes stamped "XX".
The next is #23475, not marked as to Grade but A-2 Quality engraving, and both tubes stamped "XXX". The next example is #30347, the Smith gun pictured on the cover of the Houchins book. This gun is also not marked as to grade, but was a special order and is described on the original sales invoice as a "Quality 7, Extra Gold". Both barrels feature the "XXX" barrel stamps in gold inlay.
Interestingly, the last example I've recorded is #20330. This gun is a Quality 5 with a SYRACUSE, NY address. Both barrel tubes feature "XXX" stamps, with one tube having those stamps in gold inlay to include the Belgium "Crown" trademark. Another interesting aspect of this gun is that it has two extra barrel sets and fore arms; both done by Hunter Arms and each can be dated by the fore arm inlays to different production periods. One of those extra barrel sets also features the "XXX" tube stamps but not in gold inlay. It's "trivial" features and differences such as these that make the study of the very early Smith gun so intriguing.