I'm fixing up an old Lotus Unique from 1986 or 7, I believe. Reasonably nice bike, it seems to me. Interestingly, according to the tubing stickers, the frame (maybe just the main triangle, it's not specified) is triple-butted Ishiwata EX, and the fork (obviously original) is Tange Mangaloy 2001. I've never seen a bike before with tubing from two different manufacturers. Does anyone know how common that was? Not important, just wondering.
By the way, the bike is huge--66 cm with a 36 inch standover, with the 28C tires I have on it now. I usually ride a 62 or 63, but I can JUST stand over it--touching the top tube but not really pressed against it. It's going to be interesting to ride--I've suffered with many bikes that were marginally too small (or way too small) but never one that bordered on being too big. First bike I've ever had where I can put the bars level with the seat.

Happens a lot with custom bikes.

On a lot of bikes, the main tubes would be Reynolds or Columbus but the fork would be hi-ten or whatever. A company would order tubing for the frame and maybe for the fork or just have the forks made elsewhere.

A lot of aluminum bikes have steel forks.

Closer to home, old steel Treks in the 5xx and 6xx lines were commonly mixed tubesets, different mfgrs. Lots of Italian bikes built with mixed tubesets, too, but probably more often using tubes from the same mfgr.

+1. lots of mid-range and upper range 80s treks had reynolds main tubes and tange or truetemper forks and stays.

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.

Ko Zieleman used to build with Reynolds frame tubes and Columbus forks.

Columbus Tenax Frame, Tange Fork, on my 86 Schwinn Voyageur.

Trek had a frameset with a CroMo fork and stays, and a 531 main triangle. Actually, many of their not-super-high-end 1980's frames were like that.

My Kuwaharas have Ishiwata frames and Tange forks and this seems to be the case with nearly every Kuwahara I have seen as well as many other manufacturers... producing smaller runs of custom forks would add a good deal of cost to the final bike and when you have a company that that can make quality forks (like Tange) in high numbers the cost is much lower.

+1 1992 Trek 950: True Temper frame, Tange fork.

It was common practice in the industry to utilize lesser grade material in the stays and forks. This reduced manufacturing costs while still allowing the manufacturer to apply the higher grade decal to the frame.

This practice could be used on bicycles of virtually any level. In the 1980s it was not uncommon for entry level bicycles to have plain gauge CrMo in the main tubes but hi-tensile in the forks and stays.

I recall one case where only the seat tube was plain gauge CrMo and the rest of the bicycle was hi-tensile. The sole CrMo tube allowed the manufacturer to apply the CrMo sticker to that tube. I'm sure most buyers thought the whole frame, or at least the main triangle, was CrMo, but if you looked at the decal closely, it stated "seat tube" in very fine print.

Some models, typically racing bicycles, also often use heavier gauge materials in the down tube and chainstays to stiffen the bottom bracket for powerful riders. For instance, Columbus SP down tubes and chainstays are often substituted in otherwise SL frames.

Any number of mixes could occur to tailor ride characteristics or costs. In the late 1980s aluminum frames with steel stays and forks were fairly common. A number of current bicycles use metal main frames (steel, aluminum or titanium) combined with carbon fibre stays and forks.