Tange was a major provider of complete forks for so many brands in the '80s, it doesn't surprise me that a Tange fork could be OEM on a frame made from nearly any other brand tubing...including Tange tubes (but frame not made in the Tange factory). In these cases it isn't really a mixed tubeset, it's a case of the fork being treated as a separate component from the rest of the frame, and sourced from outside...there are many times the builder didn't want to draw attention to that fact and tried to hide the Tange origins of the fork, here Lotus went the other way and put (or left on) the Tange decals on the forkblades.