Everything points to a 1977 model manufactured in late 1976. A '6L' prefix would be consistent with Bridgestone manufacture, while a '76L' prefix would be consistent with Fuji but both typically used SunTour/Sugino/Dia-Compe during this period,as oppsed tothe subject Takagi/Shimano. Given the corroborating date code on the Takagi crankset and the era appropriate 2nd generation Titlist rear derailleur, the components would appear to be OEM. One contract manufactured brand that would have a '76L prefix and use that headbadge rivet spacing would be Sekai. They also liked to spec long point lugs. However, offhand I can't recall any model using the Shimano/Takagi mix around this time. Doesn't look like a higher end frame. Possibly butted hi-tensile steel. What's the seat post diameter?

Edit: D'Oh! My mind my transposed the first two characters. '6L' would be consistent with a M-a-t-s-u-s-h-i-t-a serial number format, not Bridgestone,which would have been 'L6'! They did have a Panasonic Sport Deluxe model in 1977 with Shimano Titlist and what appears to be a Takagi crankset, though the catalog pictures show a claw version of the rear derailleur. I'm not sure if they used long point lugs and the badge was not riveted but it may have used pins on the backside?