I'm a little surprised there's no date back by the distributor. In any event, you know basically all you need to know to order parts for it. It's a '87-newer 1-piece rear main seal block, prolly w/o the bosses for a roller cam, so that makes it the same as the TBI truck engines of that era. Except for the rear main seal/pan gasket, it's basically the same as any 350 from '68 onward.
Some possible differences between your block and the earlier type (may not be all of them):
• You might have powdered metal rods (but I doubt it).
• The flexplate/flywheel has a counterweight to compensate for the loss of the small counterweight that the 2-piece rear seal cranks have.
• The crankshaft rear flange bolt pattern is smaller. This prevents the earlier (~'86-back) neutral balance flexplates or flywheels from being used.
• The fuel pump boss may or may not be machined for a mechanical pump to be used. In your case it probably is machined all the way.
In addition, up to 1985 the oil ring/piston groove was 'deep' (0.195" or thereabouts). From about 1986 to the end of the L31 Vortec production, the 'shallow' rings (around 0.165") were used- but there could still be deep groove pistons and rings being used from about '95-back.
And if that wasn't enough, there were metric rings (2mm x 1.5mm x 4mm) used from about 1992 to 2002 when the Vortec L31 engine bowed out. If you had access to the vehicle VIN, the metric ring engine (might be others) is VIN P.