Your block casting number more likely reads 3738813, which corresponds to a 1958-59 Chevy 261 block.
The block casting date code D 9 8 decodes as D = April / 9 = 9th day of April = 8 = 1958 model year.
The serial number pad being blank is a sign it was a replacement block (warranty or purchased to replace another engine)
The head casting number reads 3836850 from what I can see. That indicates a 1956-62 261 original head. The key to know what year is the heads date casting code, which is found under the valve cover, just inside the gasket rail on the driver side by the carb location.
Hope this helps clear things up.
Note: This info is provided if you find that the head and block dates are not consistent with the same year of manufacture. The head build year is important as there were some changes to the later 261 blocks and heads that could significantly affect engine operation. I learned that when we finally got down to the nitty gritty on my 261 currently being rebuilt. I have a 1962 261 replacement block which has the later oil passage ways for feeding the top end, but an earlier 1957 261 head that does not have the same passages. Due to changes in the internal oil passages for the cam and top end oil feed, there isn't a corresponding path for oil to go directly to the earlier head internally. To provide top end oiling, it requires an external oil feed line from the oil pressure feed port on the drivers side of the block, all the way around to an external threaded port into the center passenger side of the head to provide lubrication to the rockers. This was also a "fix" used for earlier 261's that had problems with blocked oil passages to the head which starved the rockers and caused early wear out of components.
