Hate to shoot at you Larry but the first ever appearance of the RC148 five cylinder Honda 125 was at the Japanese GP at Suzuka in 1965. They were indeed two and a half 50cc twins and produced 34 bhp at 20000 revs, they had eight gears. Bore and stroke was 33x29. The bikes used at the Isle of Man that year (1965) were also on debut but were the 4 cylinder 4RG146 models.
As an aside Honda's first ever GP victory was achieved by Tom Phillis on the 125 at the Spanish GP in 1961. Tom was an Australian who sadly lost his life at the Isle of Man. I have seen the above 5 Cyl bike referred to as an RC149 so someone might be able to clear that one up for me.