This is an engine from a 1961 Sebring car. This is a spare engine to be repaired by Ian Prior. There are lots of unusual features. It has no water passages between the block and the head. The water passes via the two elbows at the rear of the engine. I understand that it did not have a head gasket, but had the head and block lapped together with grinding paste. The cast in place water passages in the head were plugged with aluminum. No other water holes were ever drilled (including no bypass port under the thermostat).
This is a 1961 engine, generally based on a 1622 style engine block. The displacement number on the side of the block was not cast on, or ground off, because it is bored to 1588-cc. As it was entered in the 1600 Class at Sebring, and they did not want confusion by having 1622 on the side. It has the 1622 style narrower main bearings to allow for the stronger crankshaft design. Ian Prior has the factory build sheets e.g. Twin Cam conrods (no little end clamp bolt) etc. In short, it is a kissing cousin to a 1622 with a slight under bore to get into the 1600 class.
There is one (mysterious) dowel pin in the top of the block. With no other obvious function, my best guess is that it is intended to prevent the head from creeping around on top of the block with differential thermal expansion. Reason for one pin rather than two for alignment could be to nail down the front end and allow the back end to creep a bit if necessary.
In photo above you can just see the edge view of the core plug retaining bar (also shown below). This is a fairly common mod for a race engine to assure that the core plug cannot pop out. Safety wire on the hex head bolts and the (already self-locking) hex nut shows the extremes of care in preparation.