I'm a package driver in New Mexico, much of my time spent as a steward.
In the west, there is no differentiation between part time and full time employees. The only difference is the contribution amount.
To get "points," one must merely work 500 hours in a calendar year. This gives you "points" for the year. Five consecutive calendar years of "points" equate to being a vested member. To estimate your pension credit for the year, multiply your hours worked times your respective contribution rate, then multiply by the current year's multiplier (currently between 1-2%). This is what your monthly check increases by upon retirement.
Example: 2080 hours X 8$/hr contribution rate=$16640 in annual contribution to pension. 16640 x 1% (hypothetical, changes annualy with stock market and socioeconomic and political factors) equals $166.41. This would be the increase to your monthly pension check at retirement, given this hypothetical situation.
Once your hypothetical $166.41 is earned, it is locked in. Subsequent years merely add to whatever amount you have accrued, provided you have earned enough points to become vested.
This is explained more vaguely in your annual statement from WCTPT.