Any photos from front? Might be something from Netherlands.

Not yet, this one was with the wrong number on a map and decided to check. So the pic wouldn't be too blurred was waiting until it stops, but it didn't. Promise to check it in the next week.

No not ex-NL but ex Switzerland. TPV Vernier 13 or 15 (Volvo B10L -Berkhof Communo): http://www.snotpg.ch/album/albums/userpics/10013/PA255105.JPG

Why does this have a PO registration ? What does that signify ?

Martinsh - here in Latvia, vehicles has five types of plates, but just three of them are used on cars, buses, trucks. These are
1. Type A - Both plates are regular size rectangles and looks like XX-YYYY
2. Type B - Front plate is regular size rectangle and rear plate is square, as seen on this picture. Both has XX-YYYY on them
4. Type E - Front plate is regular size rectangle and rear plate is short, both are XX-YY

When vehicle is registered in CSDD for the first time, inspector decides which type of plate will fit to vehicle construction (as you can see, on this bus, there is impossible to put regular long plate between number light, so here is mentioned type B)
As number plates are not being made individually for each vehicle but are made before, and spreaded between all CSDD offices, at the moment of registration inspector (or owner) checked all available plates, so looks like in Liepāja CSDD, the only available B type plates were PO-XXXX :)

There are only two types of pates which are referred to owner: LA - Latvian military forces and RD which goes to Rīgas dome
(unofficials says, that exists one more letter combination LM, which is Liepājas metallurgs, but these are only rumours)
;)

Yes, but POxxxx isn't a normal issue is it ? HU is current ? Or does PO signify the unusual shaped plate ?

Also TX = taxi doesn't it ?

Yes, Forgot TX, which means Taxi.

Well, as I know, there is no more connection between time period and letters on plates. All depends from available plates in local CSDD office according to vehicle specs. My parents changed car in 2004, and as it came from USA, the only one available small rear reg. plate was OV-98.(friend's car got ZE-xx last summer)
The same seen here, while other buses got regular HU-xxxx some got EO-xx and PO-xxxx which are not current but are different and can fit without rebuilding.
The same in Ventspils reiss while all sprinters in 2005 got FV's Travegos got PC and PU.

By the way, big e-mail is on the way. Will double check some info and monday eve, I will finally send it to you :)