I believe the Sportster was first produced in 1957, making next year that model's 50th anniversary.
Here's a good page on the Sporster evolution:  http://www.sportster.org/history/
1957: 
[highlight]The Sportster XL was introduced as "a modern, up-to-date power plant." [/highlight]
55cid 7.5:1 Compression, hemi-spherical style heads, 90 degree valve angle, 4 cams 
Production Information: 
XL (1983 units), $1,103 
1958:
XLH introduced with higher compression and larger valves 
Camshaft gears became integrated with the camshafts, eliminating the need for a woodruff key. 
Drive gearshaft was enlarged .125 in. 
Drive gear and second gear support shaft was enlarged .125 in and had 8 splines (instead of 6). 
Choke control lever moved from left side to the air cleaner. 
Heavier clutch cover and new gasket. 
New oil resistant clutch hub seal and gearshaft o-ring. 
Two brush generator. 
Optional turn signals and 1 in over suspension. 
The XLC & XLCH (Competition/California Hot) 
Larger ports and valves 
High domed pistons 
Light ended tappets 
Magneto ignition 
Production Information: 
XL (579), $1,155 
XLH (711) 
XLC  
XLCH (239)