Although most Washington service numbers were in the high 100s, looking at a list of service numbers I have from 2004, there were some that were in the 200s, 300s and 400s:

291/292 - Heworth to Washington Circular
293 - Heworth to Rickleton
294 - Heworth to Newbottle
393 - Heworth to Harraton
493 - Waterview Park - Houghton (via Concord and Barnwell)

Going further back, the following ran in the late 80s into the 90s (all three were evening services Mon-Sat):

297 - Heworth to Galleries (reappeared in 1995 as route Brady Square - Heworth - Brady Square)
298 - Heworth - Sulgrave - Concord - Donwell - Heworth
299 - Heworth Donwell - Concord - Sulgrave - Heworth

Plus at various times:

279 - Coach Road - Concord - Galleries - Rickleton (Mon to Sat, day services operated by St George Travel, evening services by Go Wear) (1995 to early 2000s if I recall)

284 - Washington - Wrekenton - Birtley - Washington (reverse journey of service number 184)