type Buffer is array (1..32) of Interfaces.Unsigned_16;

not require a ‘Size clause? Or wouldn’t a compiler be free to place each 16-bit array item on, for example, a 4-byte boundary?

1. I understand why you use the GNAT-specific ‘Object_Size here. However this is not portable and I wonder why you don’t use the portable
Item_Size: constant Stream_Element_Offset :=
Item’Size / Stream_Element’Size;
I do not see how Item’Size and Buffer’Object_Size could ever differ.

2. At first I wondered why you didn’t use directly Item’Access or Item’Unchecked_Access, but on second sight I realized that would have given the wrong pointer type. But then – why don’t you use RM 13.7.2 System.Address_to_Access_Conversions instead of the Unchecked_Conversion As_SEA_Pointer? (I gather, the latter does exactly the same behind the scenes, but there must be some reason why this package has been provided by the ARG.)

BTW: Could you please take care in the future that the apostrophe is no longer shown as '. This extremely uglifies your examples.