hi andres, i have in the past had to remove the front glass on a lens. this particular lens had writing on a flat ring that was covering the screws that needed to be removed, just carefully peel that ring up.
i also have a tamron 28-300 but it's ring looks to me to be screwed on. if you look carefully, you can see the treads that it is screwed into. this ring has one smell hole on the outer edge of the ring. i believe if you insert a very small screwdriver tip into the hole, you should be able to force that ring to turn and remove it to see what is under it.. i have not needed to do this to my tamron lens but like i said earlier that ring looks to me to be screwed on. i would like to ask you questions about your tamron if you wish to contact me, ger.sec@hotmail.com
good luck
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