First, I need to thank Kent Milligan and his article at http://www.mcpressonline.com/sql/techtip-combining-multiple-row-values-into-a-single-row-with-sql-in-db2-for-i.html f
I think you just need to integrate type at the correct points thoughtout the query.
without being able to test, I think this would be close; but I may have missed something...
WITH numbered_sets(make, type, model, curr, prev) AS (
SELECT make, type, model,
ROW_NUMBER() OVER (PARTITION BY make, Type ORDER BY Make, Type, model) AS curr,
ROW_NUMBER() OVER (PARTITION BY make, type ORDER BY Make, type, model) -1 AS prev
FROM inventory)
SELECT make, Type
MAX (TRIM(L ',' FROM
CAST(SYS_CONNECT_BY_PATH(model, ',') AS VARCHAR(256)) ))
FROM numbered_sets
START WITH curr = 1
CONNECT BY make = PRIOR make AND prev = PRIOR curr and type = prior type
GROUP BY make, type
Perhaps we need do change the connect by to do a concat before connect by... though I can't see why this would help yet...
CONNECT BY concat(make,type) = PRIOR concat(make,type) AND prev = PRIOR curr