3rd/4th synchro clutch

Does anyone know if a FRC6996 synchro clutch, listed as for Series 3 suffix D all-synchro gearbox, can be used in a Series 2A suffix C part-synchro box?
I have both FRC6996 and the early version (FRC1758) sitting side by side, and apart from the shape and length of the teeth on the centre sliding section, I can't see any difference. I've held the later one up to all the parts it works with and it seems to fit - it's just the 6mm teeth instead of the earlier one's 12mm teeth that seem different. Looking at the wear marks on the old one, only the outer 6mm of the teeth make contact anyway. I gather the design change was to try to stop the box falling out of gear.
Any advice gratefully received - worried there may be a difference I can't see!
Thanks
Roger

The teeth on the later ones are coffin shape think westerns! The gears they engage with are shaped to receive them hence stopping it jump out of gear.
Log rovers pics attached.If you enlarge the layshaft pic you will see the 4th gear teeth that engage are different.

Thanks Blackburn, that makes sense. I think I can make out a difference in the 4th gear internal teeth in the photo. I was trying to work out how the coffin-shaped teeth would stop it falling out of gear, your explanation makes sense. Best stick with the FRC1758 then!