"compuesto por dos disco abresurcos y dos ruedas tapadoras en balancín"
After opening the furrows ("abresurcos"), and I suppose sowing the seeds, two covering/plugging wheels cover the seeds with earth.

Here are some examples:

A Revolution Down on the Farm: The Transformation of American ...
https://books.google.es/books?isbn=0813173159
Paul Conkin - 2008 - ‎History
The Transformation of American Agriculture since 1929 Paul Conkin ... In the 1980s new nonselective, or universal, herbicides (they kill all plant life) made ... the furrow for seeds and two funnels for fertilizer, followed by a covering wheel.
https://books.google.es/books?id=dZiKQ65FoDAC&pg=PA115&lpg=P...

Brevet US4116140 - Press wheel depth control for grain drill furrow ...
www.google.fr/patents/US4116140
A depth control for grain drills, planters or the like utilizing a press wheel which can ... 2,496,885 also shows a device having a covering wheel for a planter that is ..... 1992, Dmi, Inc. Agricultural tillage implement with improved soil leveler using ...


The Farmer's Last Frontier: Agriculture, 1860-1897
https://books.google.es/books?isbn=0873320999
Fred Albert Shannon - 1977 - ‎Business & Economics
Agriculture, 1860-1897 Fred Albert Shannon ... to cover the grain.10 The development of corn planters, before 1860, left only a few major alterations to be made. ... was dropped from an attached box, and there was a covering wheel, behind, ...


This last one is probably a translation, but it's taken from an actual planter manufacturer:

PRINCIPLES:
In order to do that and from the beginnings of its manufacturing, the characteristics which achieve these aims have been kept intact.
Ploughing blades, very narrow-angle double disc, covering V-shaped cog discs, fertilizer and seeds hopper (standard), very versatile for any type of soil because it employs the lever principle to transfer load to the coulters from the counterweight under the platform, stable and strong, easy and low maintenance, prepared to sow wheat, soya, corn and alfalfa, to fertilize in and out of the line.
12.4 x 28 wheels (big diameter) are placed in the same line which the 25 furrow openers sow, making it light to draw and of excelent operation in flooded areas.
http://www.bertini.com.ar/en/10thousand/

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Note added at 2 days21 hrs (2017-05-19 11:02:16 GMT) Post-grading
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Good, I'm glad ;@)