H306398 is the engine production code number.
H = 1961 std compression, 2 bbl, 215 cid Fireball V8, 150 HP, 220 lb-ft torque, 8.8 compression ratio
306398 = engine sequence number - used for warranty claims to factory (useless to us)
IHI1558892 is VIN number of the car the engine came from.
The I's are actually 1's, but that's how Buick frequently stamped them.
The number looks like it has too many characters according to factory data. Likely there's only a single 1 in the middle of the VIN.
First 1 = 4100 Special Deluxe series
H = 1961 model year
Second 1 = built in Flint, MI plant
558892 car sequence number where first '61 Special was 501001.
What has been, can be again. (Bob Wills, 1942)