Hi AINiPa . . .
As Greg has pointed out the lack of ID on the dizzy pad suggests a replacement part. And it is also important to keep in mind that the casting codes could 'hold over' into the next year or more until new tooling arrived at a particular plant - this is why one can encounter a date code that is much later then the casting code would suggest. For example in the webshots links (Greg posted) it shows a 3733950 casting with a 1956 date code. But many sources suggest that during 1955 the tooling was supposed to transition over to a 3737012 casting number . . . suggesting that these transitions are not firm.
Since your engine is likely a replacement part it is conceivable that it was in fact built in 1966. As far as I know casting 3769925 was the last version for a 261 . . . so if GM continued to pop out replacements for fleet service they would have been with this casting . . . but this is conjecture on my part.
regards,
stock49
