I recently bought a wt 540 for my G38. Dont know if my engine is the older version because I bought it as a short block - but I am guessing that it is since it was from an estate sale.
The carb fit perfectly on the spacer. As far as I know all WT carbs have the same bolt pattern. I also managed to turn the spacer around to provide a straight linkage for the throttle - this can be done with minimum effort!

I bought mine on ebay - under lawn equipment >chainsaws, from a seller going by 'michaelr'. Still havent had time to run it yet though.
Hope this helps.