I found the following information on Spencer Hoglund website:
* It appears as though no Iron frame Henrys were produced after serial number 400
* It appears as though the lever lock was introduced ca. Serial number 400
* The only Henrys produced without a dovetailed barrel are in 375-625 range. At this point,
it appears that all guns in this range lack a dovetailed barrel.
* Receivers were no longer dovetailed at around serial number 3000
* The apparent correct range for martial markings is roughly: 3000-4000+, and 7000-9400
however martially marked guns have been found as low as 2200 and as high as 10,000.
These guns are questionable.
* The butt plate changed shape at serial number 5000
* The loading block and front of barrel were numbered throughout most of the production
run, but with no apparent pattern. Guns with these numbers have been found from the
beginning of production through the 14,000 range. That said, the vast majority of Henrys
surveyed are lacking this feature. Of the over 743 surveyed, less than 10 exhibit this.
* It appears as though the screws were only numbered on guns up to serial number 5,000.
Sufficient information is still lacking to draw a definitive conclusion. (Plate screws through
5000, stock and tang screws through 3000.
* Of the surveyed guns, now roughly 743, 30% are engraved.
* Total surveyed 371, directly, and 743, indirectly
* 12 duplicate or known forgeries have been uncovered.
* 80% of guns have been found with sling swivels. Only 3 % had swivels on the right side,
and no right side swivels were found after roughly serial number 1300. All swivels were
rare on earlier guns.
*
Here is the link:
http://www.spencerhoglund.com/HenrySurvey.html