i'm late here. it's very hard to see the detail on the pic, i took the liberty of increasing the brightness, but a closer view from the top would be better.
i have a Cereus peruvianus 'Monstrose' and it looks very different from yours. i went thru some pains trying to ID it, so i am pretty sure about it. i also found a great specimen in bot. garden - see it in DB.
mine only doubled in size in 2 decades to whopping 30", but it's indoors and it has much less light then in conservatory. so the height variation can be tremendous depending on growing conditions.
there is cereus 'Ming Thing' that is supposed to be a dwarf variety 12"-18" - and it's been sold in the past few years in BBS. i've also seen 'Ming Thing mini' mentioned. but they all look different from yours.
while i was trying to ID mine i came around this list: may be you can go thru it and look at pics to see, if anything matches yours?
Additional valid synonyms are:
Cactus peruvianus
Cactus repandus
Cephalocereus atroviridis
Cephalocereus remolinensis
Cereus margaritensis var. micracanthus
Pilocereus atroviridis
Pilocereus remolinensis
Pilocereus repandus
Pilocereus russelianus subsp. margaritensis
Piptanthocereus peruvianus
Subpilocereus atroviridis
Subpilocereus grenadensis
Subpilocereus margaritensis
Subpilocereus remolinensis
Subpilocereus repandus
Subpilocereus repandus subsp. micracanthus
Subpilocereus russelianus var. margaritensis
