Thanks for the references
For those interested, I eventually purchased NGK 2649 YE01 glow plugs from JEGS High Performance in the USA for $22.09 each delivered.
Given I needed eight (8) plugs, that is a saving of ~$800 on the Volvo list price.
The original 'Volvo' plugs are NGKs with a different number; the specs, thread sizes, pitch
and length of the NGK 2649 are identical to the originals. What data have I missed that might bite?