PE 400 is NLA. if your lucky you may find a NOS one somewhere but generally most guys cant find them. get the PE 250 T/ RM 250 T one and cut back to second groove,( i think about 10mm, there is kind of 2 hose clamp grooves) the PE 400 boot is just a bit shorter due to larger engine in the same frame as PE 250 which spaces carby back further.
Part # = 13881-40410
Part Description = TUBE,AIR CLEANER OUTLET
Model Count = 7
PE250T 1980
PE250X 1981
PE250 Z 1982
RM250-T 1980
RM400T 1980
RS250T 1980
RS250X 1981
it was probably one of those items that Robs Rubber was going to make or had already made