dude this makes me feel stupid , because it seems that there is no translation to that particular decoration to the uniform, the only thing that i can called in english is a "TAB" that others have attributed to the collar and sleeve decoration to spanish uniforms and german too,. i found that in the spanish army including the armada they're used to denote a royal recognition for meritorious service . there seems to be no direct translation to english , so i will make the assumption (at the peril of making myself an ass) that this accoutrements to the uniform were inherited by the mexican army from spanish times as they did not have uniforms of their own for a long time and as the progression of the same incorporated spanish styling until regulations mandated changes that eliminated some of the spanish influence and adopted some french, then german and the more lasting to the last century and currently american influence, very few countries made their own uniforms that one way or the other endup being the model for hundreds of others given to the power that they exercised at the particular time in their history.
we can conclude that "SARDINETAS" is a latin american term of distinction to denote a particular specialty or expertise , but this does not apply to the spanish armed forces as it still represent the same it did before, a royal sanction.
i hope i was able to help with your question, "paisano"