Oviedo Model 1893
The Oviedo Model 1893 was made from 1896 to sometime between 1928 and 1941 depending on which source you care to believe.  Distinguishing characteristics of the Oviedo Model 1893 are:

1)  Oviedo logo consisting of crown, "Fabrica de Armas Oviedo" and the year of manufacture on the top of the reciever.
2)  Most have 29" barrels.  Some examples with 22" barrels have been reported; these appear to have had their barrels shortened and fitted with new front sights, presumably by an importer.
3)  Folding leaf, a.k.a. "flip up ladder" rear sights.
4)  No thumb cutout on the left side of the receiver rail to facilitate cartridge charging.
5) Gas escape holes in the left side of the receiver itself appear in about 1921, the second production period..
6) The bolts have a flat surface on the bottom of the face, which was supposed to provide more reliable feeding.
7)  Original bolts have straight handles
Years of Production/Serial Numbers: 

1) The "regular" series:  The earliest observed Oviedo Model 1893 has an 1896 date, and a three-number serial.  From that point on, the serial numbers follow a regular pattern of a letter followed by four numbers.  After they reached "Z" in about 1912, the series apparently continued with the number "2" preceding a letter, then four numbers.  My estimate is that about 500,000 of these were produced, based on the ones I have recorded.  . 
2) The "RE" series:  These have a plain four digit number or alphanumeric combination (e.g. A1234) preceded by "RE".  The ones I have seen so far have dates from 1928 to 1933.  The latest information that I have received (from Javier Sanchez) indicates  that "RE" means "recompuesto entubado", or "refurbished entubed", which would mean that the rifle had a barrel sleeve inserted when it was refinished.  They must have been old rifles that were reserialed and recrested when they were refurbished.  My guess is that production of regular Model 1893's at Oviedo ceased in 1928, and old rifles were refurbished from then on.  My estimate as of today is that about 18750 of these were produced/refurbished.  These are not shown in the production graph.

Note that there are some serial numbers that do not fit the pattern - the "scatter" on the production graph.  I think they were reserialed at some point, but there is no pattern with the year of production, so I assume they were reserialed at a later time, and the original markings were not changed.  Or perhaps the Spanish version of Homer Simpson was stamping serials when they were originally produced?  I simply don't know.
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Photo by Jean Plamondon, used with permission
John Robb photo, used with permission
Ed Burgess photo, used with permission
Page Updated
on 02/27/13
Serial Numbers
The production graph on this page shows production of Model 1893 Oviedo Mausers by date correlated to serial number.  Cumulative production number of a given rifle is calculated from the serial number, e.g. serial number X1045 is rifle number 241,045.  I did not make any attempt to remove out-of-sequence serial numbers; the data are presented "warts and all".  What the graph shows is that there are two production periods - through 1916, then starting again in 1921.  If you look at the serial numbers page, you can see that there is only one from 1917, and none from 1918 to 1920. I assume what happened is that production was suspended in early 1917 about the time that production of the Model 1916 began, then started again in 1921, with enhanced production capability. 
Major Variant
While most Model 1893 rifles had the standard 29" barrel, a quick look at the serial numbers page will show that there are several with 22" barrels.  I believe this was a sporterizing job involving shortening the barrels to about 22" to make these rifles more saleable in the U.S.  Observations are as follows:

1)  All of the short-barrelled Model 1893's have a step milled or ground into the end of the barrel to accomodate the front sight band, just like a normal Model 1893..   
2)  Measurements taken on a 22" barrelled example that I own show that all dimensions except barrel length are identical to a standard 29" barrel.  The rear sights of all examples reported to date are the folding leaf sights found on the 29" barrelled Model 1893's.
3)  The new barrel crowns added to the cut off barrels are professionally done as opposed to merely cut off and ground/filed straight.
4)  All examples reported to date are sporterized.  No short barrelled Model 1893's have been reported in original military configuration.
5)  Most examples are painted black rather than blued.  Far cheaper to paint than re-blue.

For more details including pictures, see the
Odds and Ends page
Production Graph
Note the button on the left side of the sight, which indicates Oviedo manufacture (of the sight, at least)