The 4903 was the first of GM's 40' "Buffalo" models, along with it's 35' sister, the 4107.
As Clifford mentioned, only 400 were built in '68 > '69 before the model was superseded by the 4905 in early 1970, which is basically the same coach. The 4905s are far more plentiful, as they were built right up until the end of GM highway coach production in July of 1980.
Two of the major differences between the 4903 and the 4905 is the far better OEM dashboard and Sheppard steering gear, which also tightens up the turning radius a little. Doing a Google Images search for the two will show the subtle evolution between models.
Biggest baggage bins of any 40' coach, as well as the longest wheelbase @ 318". As a matter of fact, the coach has larger baggage bins and a longer wheelbase than several of the 45' buses on today's market! Altho the large turning radius can be a bugaboo in certain situations, the coach rides like a dream on it's long wheelbase. IMHO, far more stable in crosswinds than it's 35' sister, too. BTDTHTS!!
One reason the Buffalos have not been as popular as conversion coaches is due to the "stadium seating" in the first four rows. Because of this arrangement, unless you level the floor, you lose valuable space.
OEM from the factory, manual gearboxed 40' Buffalos had a slightly lower rear axle ratio than the 35' models (4.375:1 vs 4.125:1). This gave the coach similar acceleration characteristics, but for any given speed, you were turning about 100 rpm more. Thus, lower top speed and lower fuel economy (altho better than some MCI models). 6 > 8 mpg is normal for the stick shift, YMMV with an automatic.
Automatics were not available as options from the factory until late '75 or so, and very few were so equipped. Any earlier vintage with one, be it a VS or V-730, is a retrofit. Look for a cobbled bulkhead opening on the curbside, and check for reinforcement.
Starting with the 4106, all of GM's highway models came from the factory with 8V71N engines tuned for 275 hp. Regardless of the model, that meant 35 mph on the governor in 2nd gear climbing 6% grades. (Same for Eagles, MCIs & Prevosts, for that matter!!) If you don't try to drive it like a BMW, you'll be happy. Think Aesop's Fable of the Tortoise & the Hare for the proper mindset of a coach pilot.
FWIW & HTH. . .