That may be a Chevy internal "build work order number" as discussed in Wayne Guinn's book "Camaro Untold Secrets". The 5-digit number could be the account number assigned to a budget for the particular development of a part. The "F" might stand for the 6th example. Since this is a '67 block but you say it has a spin-on filter pad, that may have been the whole purpose of this block. The spin-on filter was new for '68 and '67-earlier block were using the canister style filter. You don't show the engine valley but they may also have cast it without the breather system which runs out the back, hence something else to test out. Those are my thoughts based on what you have shown so far.