I'm flying this Sat. from ATL-EWR via ORD (could've taken Atlantic Coast, but they're the people who got me the $125 voucher from CS in the first place .) My flight out of ATL, UA 1781, shows a/c type as Boeing 737, without the dash number indicated. Does this mean that I'm on one of the ancient -200s, or that the particular 737 model is still unknown by UA at this point?

N.B. My ORD-EWR flight was listed as 737-500.