I don't know, but I suppose there is always a chance that Tamron don't actually make them and another company do the manufacturing for them: is it perhaps another case of step forward Cosina, or some unknown Chinese company. The Cosina Files on here gives an idea of the not inconsiderable spectrum of Cosina manufactured photographic equipment.
I occasionally use a matched Canon 1.4 multiplier on my 70-200 f4L lens. There is almost no degradation of the image, unlike the older (and considerably cheaper) tele-converters from the 1970s.
Tamron usually make their own equipment, and the film period convertors were nearly always good. Convertors in general suffer from hand shake, the doubling halves the definition, but most lenses can take this, but a 135mm on a convertor is a hand full to hand hold, ruining the definition. Digital comes to the rescue with image stabilisation, a 135mm on an m4/3 Olympus is doubled before a 2x, but is hand holdable, and still with care, with a convertor on as well. A monopod is your friend with tele convertors in general, Stephen
The problem is not so much holding a 135mm on a converter but: 1) there is a penalty in the exposure - a 2X converter loses 2 stops 2) the lens is now effectively a 270mm so there is a need to ensure the shutter speed is fast enough for that.
In effect, then, there is a three stop penalty: two that the converter uses up and another to increase the shutter speed.
It's not just a case of decreasing the definition with a converter, but also increasing the aberrations. This is twofold: magnifying the aberrations of the original lens and secondary introducing its own.
Digital has the added advantage of being able to increase the "film" speed on the fly.