The early EMCO Maier supplied collet holders were for the E-16 and ES-6 series of collets from Schaublin in Switzerland and were a little shorter than the ER-16 style invented by Rego-Fix (also of Switzerland) in 1972 and do not have the ER-16 extraction groove (ES = Schaublin, ER = Rego-Fix).
An ER-16 collet can sometimes be used in an E/EX-16 holder - but never the reverse i.e. an E/ES-16 collet in a ER-16 holder - if this is tried the extractor in the nut will ruin the smaller collet. Although the ER-16 collets will seat in the E/ES holder they will protrude more and the nut may not have enough threads to engage the body safely. Schaublin now makes an ESX-16 collet set with is compliant with the DIN standard.
The current M14-1 Collet holders are for the Unimat 3/4 and use the ER-16 collets. The ER-16 collet and the rest of the ER family were invented by REGO-FIX . The E/ES-16 series became obsolete in mid 1980s..
Unfortunately there are several designations for the ER collet family (including .RD) with different makers branding collets with their own nomenclature. From a user's point of view, ask the manufacturer if the collets he sells are the same as the ER family - and meet the design requirements of the DIN specification. The E collets have just 6 slots and, with a collapse range of only a few thousandths of an inch, are intended for use only with the "nominal" size marked on the collet.  The ES and ER collets have 8, 12 (or more) slots and a collapse range of about .040". ER collets also have an extraction groove-- the E and ES collets do not. The diameter of the E and ES collets is measured at the largest diameter where the two cones meet which, in the case of E/ES-16 collets, is 16 mm. The ER type are measured at a defined distance below the extraction groove on the 8-degree (16 Degrees included angle).
An ER-16 collet can sometimes be used in an E/EX-16 holder - but never the reverse i.e. an E/ES-16 collet in a ER-16 holder - if this is tried the extractor in the nut will ruin the smaller collet. Although the ER-16 collets will seat in the E/ES holder they will protrude more and the nut may not have enough threads to engage the body safely. Schaublin now makes an ESX-16 collet set that is compliant with the DIN standard.
The E/ES-16 series became obsolete in mid 1980s..