Good morning, I picked up a Manco thunderbird roller. Was told it’s a 1975 but seems all the ones I can find online are 80’s 90’s. The frame only has one rear shock mount hole on frame (not 2 like all others) and no rear sprocket guard on frame ( was not removed). Has old style serial number tag with just serial numbers no model. Trying to identify the vintage. Any info would be greatly appreciated. Can’t find any info online. I’ll try to attach pictures. Started restore and mods.