1894 El Salvador Cover

I just listed on ebay and noticed..the embossed image on the back of the cover?

It seems to be the embossing of the center of the postage marking. This would mean this envelope was made by applying the printing and embossing of the postage after the envelope was cut and glued together. From the way the flap covers a portion of the embossing on the back, it looks like these were made by putting the pre-made envelopes through the printing/embossing machine with the flaps in the up position.

I do not know El Salvador postal stationery, but I would guess this was the standard way these were made. I have seen postal stantionery from some other countries made in the same way.