The Perkins/Volvo equivalents are always confusing to me, but I thought the Perkins Prima M50, which Amel fit on some Santorins (including mine) was equivalent to the Volvo MD22, both of which rated 50 hp at 3000 rpm That's the MD22 I think Herb has on his Santorin.
I had thought that Amel used the Perkins Prima M80 (a turbo M50) on (some of) the SM's for 80 hp and also the Volvo TDM22A turbo rated at 79hp. There was also a Perkins Prima M60 at 60hp and a Volvo MD22 and MD22A rated at 59 hp at 4000 rpm (vs. 50hp at 3000). Don't know if Amel used those.
Anyway, my M50 does not have an oil cooling section in the end of the heat exchanger, nor any oil hoses going to the heat exchanger, and I suspect Herb's is like that. All oil cooling is accomplished as the oil circulates inside of the block. My M50 also does not cool the ATF transmission oil.
Is your MD22 perhaps the 60hp version (also naturally aspirated), which could explain the oil cooler you describe? It still seems small for a SM, or is it the 79 hp TMD turbo?