I just purchased an 18025 large frame tiller with extension kit to make it a 33". Im going to use this on my HT24D (ht20D). When I first started looking I knew there was a difference in the lift arms between early 1250, 1455 etc vs 1886 01-04 vs HT series. I wasn't looking into it too closely and just thought the lift rods were different lengths. As i really started researching this i found that the lift rods are all the exact same part number but its the lift arms that bolt to the tiller...
Large frame tiller lift arms
Large frame tiller lift arms