M is GM of Mexico in the number designation but the next 4 numbers should be the date of manufacture as in 0105...January 5th. Then there should be a three digit suffix designation. I would check the number again very closely using a wire brush and chalk to highlight the digits and letters.
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'68 C20 Longhorn 50th Anniversary 400/TH400
'68 C20 Longhorn 50th Anniversary 468/TH400w/buckets
'72 C20 Halfhorn (Longhorn w/o cab and front clip)
'69 Flxible Cruiser (look up ugly in the dictionary)
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