If yours came with a 2" diagonal and focuser then it has probably already got the standard SCT thread. The ETX has its own flip mirror system and a photo port at the rear of the tube. The port has the standard T thread (M42 - 0.75 or metric 42mm with a 0.75 mm pitch). From memory the meade SCT thread is imperial and a little less then 2" (~ 1.997") with 24 TPI (threads per inch). Celestrons are similar I believe
. Now the large visual back on the meade lx200 series is 3.5 inch with a 16TPI. Realisically you will need to find a crayford manfacturer who also makes visual backs suitable for your scope. I know Moonlite will do a custom job for you if necessary but it will cost plenty and I don't think it would be worth it on a 127mm scope. All the crayfords I have came with a adapter ring to allow mounting of the focuser (usually 3 bolts into a milled groove). Hope this helps but I fear it may confuse you even more.
Mark