Jumping in my way back machine, I recall that the 3630 was the same as the 3650, but the number was different because Compaq used different numbering to track retail vs. corporate sales locations. HP still carries on this confusing tradition, but only varies the last digit. The 3670, as far as I recall, had more internal memory (64 megs vs. 32). Otherwise it's identical to the 3650, including serial port support.
The slightly newer iPAQ 3800 line also supported serial via the sync connector. But the connector itself is slightly different on the 3800, so if you have a custom iPAQ 3600 series cable from Trimble it won't work.