Kind of doubt a custom built frame and fork with forged DOs, forkends and the frame bits this has would use plain gauge 531 in 1979. Given the size and the fact it had eyelets and used 27" wheels it makes sense the tubing is beefier than "a road frame" would have but, if the serial number "104" does indicate the frames built (total so far or in the year '79) this was not a hobbyist. The selections made (like tubing best for the bicycle's job) would have been deliberate.