A little history is in oder here. GM built the TDH 5300 series (fishbowls) from 1969 to 1977 in the US. At that point the RTS was introduced. Gm in Canada, known as Detroit Diesel LTD, however continued to build fishbowls into 1983. At this point DD LTD reconfigured the fishbowl giving it a new front end and a new side window treatment and continued to sell them for several more years. DD LTD never produced the RTS. Many American cities bought Canadian fishbowls as they were cheaper and easier to maintain than the RTS.
PS The model designation for GM buses was the first letter stood for the type of service:"T" stood for Transit, "S" for Suburban, or "P" for Palor (over-the-road); The second letter stood for engine type: "D" for diesel, "G" for Gas; the third letter stood for transmission type: "H" for Hydraulic (automatic), "M" for manual shift. and "E" for Electric. The first two number stood for normal seating capacity and the last two denoted the series model. So a TDH 5301 stood for Transit, Diesel, Hydraulic, 53 passenger, first in the series.