Reverse lock on early Kingfisher

[afarmery]

Please can someone explain how the thrust block reverse lock works on my 1987 Kingfisher? The system isn't the more recent one (where the thrust block lock is connected to the gear shift rod), but is just a simple caliper with an open section that just fits around a rubber sleeve/bush around the drive leg. The caliper can be lifted and it swings on a hinge, just clearing the upper tapered part of the rubber sleeve, thus allowing the motor to be tilted right up. However, when the motor is dropped back down, the caliper swings back over the rubber sleeve locking the drive leg. This being the case, what happens if you hit and underwater obstruction of scrape the bottom? The motor won't be able to swing up safely if it's permanently running in the locked position. THanks for any advice. (The attached picture is just a stock photo of a similar motor with the same reverse lock mechanism)