The screw in question on "your" engine model/year alone is used in a few other places vs just the rocker cover....They are used on the front cover and the oil pump screen assembly as a quick example...,
http://www.mercruiserparts.com/bam/subassembly/30905/4893/210
That same part number is also used on a very high multiple engine models in many places....Mostly for the front timing cover but still used in other locations as well for the same engines.....Anywhere between 120HP to 454 big blocks.
There is also another Mercruiser part number that is considered the same as the 10-35366 screw in question which is 10-8M0040582 found as an superseded type part number and that other number offered a size which helps locate the size info you're looking for under the 10-35366 number. Both are listed as,
10-35366 SCREW, (.250-20 x .563)....And,
SCREW, (.250-20 x .563), CRANKCASE FRONT END COVER TO BLOCK (7/16") (#10-8M0040582)
Item # 21,
http://www.mercruiserparts.com/bam/subassembly/31837/2697/190
While part number 10-8M0040582 is a lower cost to part number 10-35366, some places where saying it's a special order so it would take much longer to receive if you went the Merc route....Now that you know a few other places this bolts fit, you should have less issues finding it elsewhere such as auto parts store part computer searches if needed.
If interested to know what this one bolt fits model wise, just have a scroll through the following engine list,
http://www.mercruiserparts.com/35366-screw 
