DARREN - A quick check of the parts look-up will show the 124 has what is referred to as a 19 tooth second gear and the 70 has a 16 tooth second gear. First and third were always the same ratio therefore the same speed. You didn't mention it in your post but ONLY second gear is where Your 124 should be faster. Tire size, etc will also vary speed, larger dia. tires equals greater speed. There's also a stock 17-tooth second gear set that was available, think it was in the gear drive red tractors and maybe the quiet-lines. It's just a bit fater than the 16T but not as fast as the 19T. Relative speeds are 3.2 mph for 16T; 3.9 mph for 19T, and I think 3.4 mph for 17T, all with stock 23 inch dia. tires and 3600 rpm.
I have a 70 and a 72, well, actually the 72 would be a 144, and second gear in the 72 is actually too fast to mow but a really good gear for things like idling around with a cart, pulling a moldboard plow, lawn areator, but I have to mow in first gear. The 70 however will become my preferred mowing tractor when it gets a rebuilt K241 in it, that slower second gear is a GREAT mowing gear.