101 is a short bed 1/2 ton GMC. The lack of the number 8 after the 101 says it was born with a six cylinder engine, not a V8. The P is manufactured in the Pontiac Michigan plant. The S is model year. Unfortunately GMC used S in both 58 and 59. I know how to tell the difference if a V8, but not a six. I'm guessing (because of the fairly high 33517 serial number) that this is a '59 truck, not a '58, but I do not know the break point between 58 and 59. Check the casting date on the top surface of the head for a three or four character date code. It will look like F28 or B139. The letter is the month of the year the head was made, A through L for January through December. The last number is the last number of the year, ie. 8 for 1958, 9 for 1959. The one or two digits in between are the day of the month. So using my examples F28 would be Jun 2, 1958 and B139 would be Feb 13, 1959. You can also find a casting date on the driver's side of the engine block. No guarantees that the head or the whole engine hasn't been swapped in the 60 years since it was made. Not to belittle this web site, but you'd have a lot more luck with GMC specific questions at OldGMCTrucks.org Oh, and welcome to our forum.