I have a road bike frame with a serial number of 80 03 51516, does anyone have any idea on how to identify this bike. My guess with the number is the it might be a March of 1980 production. It had a mix of Shimano 600 and Campagnolo Strada and Record mixed up on it. No stickers, no other branding. Would love some help with this!
You're pretty much out of luck, unless some part of the bike is particularly distinctive. We see this a lot with both generic Bike Boom road frames from the 80s, and BMXs from all times.
If your bike has a distinctive feature, then please edit a clear and well lit photo into your question. Ideally one of the whole bike from the right hand side, and another of the part.
The occasional trend of repainting a bike and removing decals makes it hard to identify - curse the previous owner for that choice! If you know the history of the bike, then ask the previous owner/s if possible.
That your serial number is 4, 5, or 9 digits long suggests that it is a larger manufacturer. Small bespoke shops would not produce 999 frames in a year, so 51516 suggests a volume output.
You can also identify the exact model of the components, and see if they have an overlap in dates, and make a judgement call if they are original or not.
Example: Shimano 600EX 6 speed duraace was new in 1983, while 600 Ultegra (6402) was 8 speed SIS with STI levers and dates from 1992.
Info from https://en.wikipedia.org/wiki/Shimano#Road_groupsets, and
http://velobase.com/ViewGroup.aspx?GroupID=fdd9d3ff-38c7-439c-b9c7-e28c90d1ef87 has more info on each item.