Comparing pictures of your block to my 153 and the only thing I see that looks a little different is maybe the machining of the lifter bores. Could be the picture but it looks like your bores are moved off to one side of the casting compared to mine
I thought about that too but It looks like they are shifted in the opposite direction and the spacing is wider than the Indy configuration. Maybe measure the spacing and more pictures straight over the lifter bores
They ran a few different lifter configurations on the Indy engines. There was one version with siamese lifter bores where the lifters were keyed as an anti rotation feature. This is like the first pic Rollie posted. There is a 2nd version that ran siamese lifter bores with a link bar set up but they were limited on lift due to the short link bar. Maybe this is the last version where they split the difference between the siamese lifter bores and the stock lifter location.
If you look at my third pic you can see in the front cam bearing area there is a key way / groove cut for oil transfer. The rear has a similar notch also.
I’m in the thinking it’s a 153 with a few extras, but why change the casting number?
I’ll put up a couple new pics of cam bearing areas.