The TT250/350 did really have a bash plate as standard. there was a pressed steel bracket "thingie" that was mounted between the frame rails to protect the sump area of the engine, and a set of engine case protection bars that provided some protection for the Clutch and Flywheel covers.
For some reason you dont find many of the engine case protection bars around, I dont know if it was a case of people taking them of so that they could get one of the engine covers off, then not bothering to refit them.
For the TT250 that I have, I made my own 5mm aluminium sump plate that bolts onto the same brackets as the original sump protection thingie, and was able to find a engine case protection bar on eBay.